I do agree with being able to restrict your civilian shipping in a way. However, it is supposed to be a private sector out of the control of the government, so instead of being able to limit it directly I think it should be indirect restrictions set up like conditional orders kind of like what you have already but a little more in depth. Like have conditions to the age, technology, and number of civilian ships and have them (shipping lines) pay higher taxes exponentially the more ships it has and the older technology it uses or have them to pay higher maintenance for older ships, but have this to be set up by the player.
Fully agreed that it should be handled indirectly. In reality government can always regulate private enterprises via subsidies and taxation.
We know that currently a shipping delivery ( of 10 goods ) will grant the shipping line 20 credits, and the government(s) will get an additional 10 in tax, So the total value is 30 and tax 33%.
Since there is zero running cost currently I will also assume this is abstracted away so that the 30 credits is actually the profit after running costs are deducted.
I suggest two levels for this control:
Global taxation:We can during the game set the global taxation level for each shipping type ( trade goods, colonists, fuel ) and this also affect the likelihood new ships of those types are created. 100% tax = 0% chance this ship type is chosen for new construction ( but trades will still run since 100% tax just means 100% the profit is taxed ). This also gives you some long term control of what type of shipping you want to emerge in your empire dynamically depending on needs. If you think shipping is growing out of hand simply put all to 100% tax and they will not be able to afford replacing old ships and will start to decline. You can also put a negative tax percentage to show specific subsidies ( -50% tax on trade goods means each run of 10 goods will give the shipping line 45 Credits, 30 from the run and an additional 50% in government subsidies ), and this also means that civilian freigthers have over twice as high chance to be selected for new construction compared to other ship types that have normal taxation ( x1.5 modifier instead of x0.67 ).
Local (systemwide) taxation:To promote traffic to a certain system ( most of the cases we want to control the destination of colonists and infrastructure, not the origin ) you can also addatively modify the tax rate in a system from the Civilian/ Ind Status tab. For example I want to boost traffic to my new colonization target so I set the tax modifier here to -20% which will reduce the standard 33% ( if that is where the global tax is set ) to a mere 13%, meaning the shipping line will earn some extra credits and be more likely to pick this destination at the expense of the government budget.
This way I think we could solve the problem of controlling size of shipping lines, but in a more interesting and realistic way, while also adding alot of other nice and often requested features like control over where shipping lines are going and what kind of ships they are building!