Are colony rebellions enabled yet or is that mechanism still being worked on?
So far you have to do it manually through independence. I have played a lot with the mechanic recently and I have understood why is soo hard to implement. out of 5 attempts to just declare independence and change the DB so that the new faction would be led by the AI, only 1 succeded and I still don't really know why or well I guess that the reason was for me to leave it as it was. The problem with that is you have a completely worthless opponent. All the others that involved some sort of help like ships or troops have either: broke the game after a few decades or simply thrown multiple errors every sub pulse click making it unplayable.
I think there is some sort of heavy coding involved to actually make this a mechanic. It would require the NPR to be generated as it was brand new but I don't think people will be happy with a bunch of ships or troops appeared out of nowhere. Unless you happy with just a planet where you need to send ground troops unopposed...
I don't think rebellion has to be a planet declaring independence though. You could have riots and stuff that fire once unrest has been high for a long enough period of time. Installations are destroyed, ground units take casualties, missile stockpiles are rendered useless. Maybe even things in orbit are sabotaged, damaging or destroying ships etc. Unrest could even spread to other planets to the same system and to a lesser extent planets that are in the same sector (based on no. of jumps from the unrest).
To me this sounds like an easier way than implementing some form of secession mechanic.
Declaring independence feels like it should be related to a more novel secession mechanic that is maybe handled on the sector command level (yes I know you could game this by not having sectors). When a sector as a whole has high unrest across multiple worlds/systems it might decide to secede from the empire with planets determining allegiance based on how big and stable they are. If you feel like putting in the work you could also factor in the industrial dependence of a colony to the rest of the empire when factoring this in. This could make it so that instead of individual colonies, entire systems or sections of the empire could secede and become a real pain in the ass as opposed to a lone planet rising up on its own.
I don't know how you'd allocate ships and space installations. It makes sense to make engineless ships switch/stay on the side that their planet has joined. I think its acceptable to spawn somewhat weak but numerous infantry based militias to act as the initial garrison to the new independent worlds depending on population size. You could make the homeworlds of officers as a factor, making officers more likely to attempt to defect their ships to their home sector / planet if its declared independence.
I think for the time being a simple rebellion planet state would be good to at least provide RPers with a prelude to manual independence and to provide players more of an incentive to police their worlds.