I tend to agree with everyone else that just rescaling the profit/loss for shipping lines won't solve the problem, just push it down the road a bit. That being said, I do think some suggestions here are a bit broader than needed to accomplish the stated objective:

So I am considering changing how shipping lines work, so they don't become so overwhelming, while trying to retain the flavour.

emphasis mine.

In my view (and from reading comments here), CSLs become overwhelming when they have too many ships - this unbalances the economy (due to tax income), trivializes new colony growth, causes rapid (de)population of colonies which disincentivizes use of source/destination automation, and causes performance problems (although this was mitigated by the change to use only larger ships).

Why do CSLs build too many ships? Simply put, because they keep making money and have no other way to use that money - there are no other money sinks for CSLs. This brings us back to:

Another option is replacing dividends with a percentage maintenance payment, which is modified by an admin overhead that increases as the number of ships increases - effectively limiting the rate at which a shipping line can increase in size.

I think this is probably the best starting point - as the saying goes, the bureaucracy is expanding to meet the needs of the expanding bureaucracy, so too goes the shipping company.

I would suggest implementing a civilian maintenance fee with superlinear scaling per ship. If N is the number of ships, something like N^(3/2) or maybe even N^2 (maybe this is too much) should work. If the rate of profits depends roughly on N but the rate of maintenance depends on N^(3/2), for instance, then there will eventually be a hard limit on the number of ships a CSL can buy and operate. This also means they will grow more slowly as they come closer to this point.

I would also suggest that CSL income is affected by the wealth generation tech level of their associated race (while the maintenance fee scaling remains the same throughout the game). This would mean the limit on number of ships is initially lower but increases with tech level, which is a rough analogue for the growth of the player empire.

I think this is the simplest and best approach as it is more or less completely under the hood (except for tying to research, which is only a positive change IMO). The essential flavor and interaction with CSLs otherwise remains the same, they just become more manageable. I don't like, for example, ideas about tying CSL growth to number and sizes of populations, I feel like this creates a danger of trying to micromanage populations to manipulate CSL growth (whether to speed up or slow down), which is not a style of gameplay I think fits Aurora.

----

There are a few other suggestions in this thread which I think merit consideration. They do not address the issue raised in the OP, at least IMO, but they are good ideas anyways:

Consider that civilians just use 50% power reduced engines and keep the higher speeds of those ships.

Agree as this makes the minimum engine power tech more attractive and will contribute to limiting CSL growth rates due to higher costs per ship.

--- Perhaps give them a sort of fluff base? Like CMCs, they'll spring up on eligible worlds, but would serve as mock "supply bases".

This would probably be too much work, but I would highly approve of CSLs having to build/expand their own shipyards for example. Being able to pop a 200,000-ton freighter into miraculous existence with no orbital infrastructure is a little silly.

At the very least can we get the oft requested reserve/target min/max settings for population, similar to whats already in place for minerals and installations, so that our source colonies aren't perpetually evacuated if left unattended.

It is oft-requested, but I would like to add my support here. In games with multiple player races, trying to keep track of all source/destination populations across multiple races is tedious at best and impossible at worst, for a single player race maybe it is "more immersive" not to have this automation option but for multiple player race games I think it is essential. As it is, I turn off civilian shipping in multiple player race games to avoid these problems. Implementation could be simple: have a trigger population value set for a source or destination colony, and when it is reached set that colony as stable - no or minimal changes to the civilian AI are needed.

This being said, these suggestions mean potentially significant mechanical changes and a lot more work on the coding side, so while I like them I wouldn't say they are necessary to solve the problem.

It seems strange, but searching for a similar request yielded no results: would it be possible to implement an "autorefresh" function for open/background windows? I think it would benefit the QOL.

-J-

Hey Buddy,

To implement an "autorefresh" function for open/background windows.  You can use browser extensions like "Auto Refresh Plus" for Chrome or "Tab Auto Refresh" for Firefox. 
These extensions allow you to set intervals for automatic page refresh, improving your quality of life by keeping content up-to-date without manual intervention.  Just install the extension, configure the refresh interval, and enjoy seamless updates on your open tabs.

Thanks,
James

I would like to see some deviation from the 1:1 freighter:colonist transport. I like civilians moving around doing stuff but what normally happens is I start marking colonies as "stable" to keep the growing fleets from draining the population sources and then the colonist fleet sits largely idle somewhere while the freighters keep moving trade goods, but the shipping company keeps manufacturing colonist ships (just in case?). That just makes the problem worse later when I open up a destination and suddenly millions and millions of colonist capacity gets directed to a single location.
I would personally be content with just a different set ratio, such as 2:1 or 3:1, but there are probably better solutions that would make everybody happy. Maybe the company occasionally checks for idle vessels and scraps one, and then at the same time launches a vessel of the other type. Or when it's time to construct a new vessel the company checks how much money it made in the last year on colonist vs freight transport and builds whichever type made more money (with some chance to build the other type anyway, lest the system run away fully to one type or another).

Also I think colonist transport shouldn't generate tax money for the player. It doesn't make sense that I accidentally tell 50 million people to move to Mars, getting paid the whole time, and then when I realize my mistake and have them move back to Ganymede to work in the mines I get paid again.

To expand on some of your ideas:

Rather than simple source/destination/stable options and the heavy-handed colonist movement that occurs as a result of the massive colony fleets, allowing numeric configuration of a target would be useful, i.e. "source until x mil" and "destination until y mil" options.

Leveraging your idle vessel idea (I agree that nearly lockstep 1:1 should be deviated from):
- Idle vessels could also inform the company's decision of which hull to build as well as scrap so that they never over-build in the first place.
- In a relative sense, if colony ships have a lower percent utilization than freighters, the company could be more likely to build a freighter than a colony ship and more likely to scrap a colony ship than a freighter to balance the two utilizations. No sense building or maintaining ships in a market that's already saturated; better to expand where there's availability.
- In an absolute sense, a low percent utilization could lead to less construction and more scraping while a high percent utilization could lead to more construction and less scraping in an effort to more effectively use the existing tonnage. No need for the company to waste wealth building or maintaining vessels that aren't used.

On an entirely different note, while civilian gas harvesting can be disabled, there are no options to specifically disable civilian mining, civilian cargo, and civilian colony transport. Allowing a finer-grained disabling structure might eliminate some of the issues by removing the cause entirely.

When toggling the Lifepods checkbox on in the Naval Organization view, an error pops up and must be cleared in triplicate. I suspect it is attached to a couple of maybe-broken 0-crew lifepods in the same system as the fleet is trying to view (the two lifepods do not show up on the list of contacts, left-hand column, despite Lifepods being checked on). I generated these two lifepods unintentionally when I deleted a test (SM) fleet that contained one carrier and two hangared parasites; while the carrier was deleted, both parasites left a lifepod and a wreck instead of being properly removed.

Edit: the error no longer generates after those two lifepods expired. My money's still on something wrong with those two in particular.

Info dump:
- "2.5.1 Function #773: Object reference not set to an instance of an object."
- Generates from the Naval Organization window in triplicate when checking Lifepods on
- Possibly connected to a pair of empty life pods; see above description and below attached screenshot
- Conventional start
- Real stars
- US decimal and time formats (I have not reconfigured for UK standards)
- Approximately 83 years into the campaign
- Campaign started and running on v2.5.1

I definitely feel that some kind of rebalancing needs to take place - in year 88 of my game, the civilian sector makes up a staggering 81.8% of my economy. (38.8% Shipping Colonists, 25.1% Shipping Trade Goods, 12.1% Passenger Liners, 5.8% Exports.)

It's basically nullified the entire wealth-as-a-resource aspect of the game, since I've got an income 7x as great as my expenditures.