I've hit three total error functions recently (though not at the same time) while running time with no specific window or identifiable context:

2.5.1 Function #2788: Object reference not set to an instance of an object.
2.5.1 Function #1531: Object reference not set to an instance of an object. (This occurred two different times)

#1531 may have something to do with an NPR generating a free colony with the default free 100 population and 1 naval command after being wiped out. I've been playing whack-a-mole with this NPR trying to wipe it out completely and have been generating quite a few free naval commands as a result; the logic there may warrant a look for more than just the function error. For #2788 I truly have no ideas unfortunately.

Standard info dump:
- Conventional start
- Real stars
- US decimal and time formats (I have not reconfigured for UK standards)
- Approximately 85 years into the campaign
- Campaign started and running on v2.5.1

Editing in: I've now received #1531 nine more times with even less context than the first two times. I'm still getting them after deleting my 'blocker' colonies that seemed to prevent the NPR from generating a free starting pop of 100 on the same body. The NPR also still has no colonies in the DB so perhaps the wiped-out NPR is not relevant after all.

Editing in a second time: I've hit a different error after rebooting Aurora; #1531 seems to have quieted with the reboot, but this one is new. It came after a tremendous amount of 5-second death but provided me with no context for its existence outside of the error text itself:

2.5.1 Function #3656: The given key was not present in the dictionary.

To move a surface mine, you need 25kt of cargo hold ...

True. But you can use civilian shipping lines to move installations and colonists between colonies.
Apart starting Infrastructures and Colonists, and very few mines or AMs, I let civilians do so, while my cargo ships get minerals from bodies: when there are many mines or AMs on a location, I find the minerals flow towards the industry planets is higher using cargo ships than mass drivers.

I like the idea of "Colonization Pressure", but I think there is a factor missing.
Aurora does consider the security and environmental effects, but not being able to find a work and
to earn a living has been a major factor for migration around to world.

I would therefore suggest that either:
- a high unemployment rate increases the colonization pressure, or
- the number of unemployed workers could increase the maximum number of colonists

Stating the obvious probably, but often even the obvious escapes me

The cheapest way to extract minerals: manual mines. Then, Orbital Miners, but they are restricted to small bodies (can the body be populated? AM are fine though). And finally, the costliest option, Auto-mines.

I'm asking because I'm just starting with OM and it seems like a good middle ground. Although you need tugs to move them, and they can't be refitted unless you have a shipyard for them, which I don't.

Orbital Miners are the best way to mine.
You get the same productivity as a surface mine for the same cost, but it is 20% the size and does not require population to run.

The overhead cost to put the modules on an armorless space station is minimal.
The cost of a simple 5-module station is 86.7% modules. So the cost of overhead is about 15.3% of the cost of the modules.
The cost of a 50-module station is 88.6% modules. ~12.8% overhead.

The savings is in the cost of transportation.
To move a surface mine, you need 25kt of cargo hold, which runs empty in one direction.
To tug a 5-module orbital mining station, take the cargo hold off of your freighter, and replace the cargo shuttle bay with a tractor beam.
The outbound haul will be at roughly the same speed as before. The return trip will be faster. How much faster depends on the size of your tug relative to the station. Assuming roughly 1:1 size, the return leg is at 2x the speed of the outbound leg.
You now have a smaller and cheaper ship moving 5 times the mining capability in round trips that take ~25% less time, and which do not have to be accompanied by even more ships carrying population and infrastructure.

2.  Is Improved Command and Control, I think in Logistics but it could be construction