When it comes to population growth rates, a lot of things still seem kinda unbalanced and dont really make a lot of sense... as your world grows, the population growth rates get slower and slower, even though the actual "growth numbers" might be getting much bigger. when you're sending your people off to other planets and starbases, it doesn't make any sense that everyone on your big worlds suddenly decides to stop breeding very quickly while there's still a decent population to breed from and a very large demand for more people. i know that you're trying to avoid population snowballing in the game, but there's many other ways to control that... the planet class, and housing, and food supply, and pop growth rates, and approval (which could work more like it did before, where the more population u gradually add, the more you need to add approval stuff to keep them happy) can all be used to make sure it doesnt become too overpowered, but the current system of everything just getting slower and slower and slower while there's so many more planets and resources to get means that i haven't even had to think about the entire food aspect of the game. it's never needed. i focus on approval for a core planet right away, then never need to look at it again. if feels like all these systems that could help control population snowballing aren't being used, in favor of a system that makes no sense when you really think about it.
With the current system, synthetic and silicon-based growth doesnt seem like it can compete with carbon-based and aquatic life forms either. It takes way too long to build synthetic citizens to be able to compete with organics when it comes to grabbing the best planets and resources near your homeworld before the other civs, and the silicon-based growth rate is only half that of the organics, even with 30 starting promethion. by the time u get enough resources to increase their growth rate to even be equal to organics, the organics will have already gotten most of the good planets and resources in the area. and that doesnt even include them trying to compete with a race like the mimot. here's a few ideas i had for each of those life forms that might help them compete with the organics...
Yor: I love their starting world. the durantium cloud does a great job of starting them off with a resource they require to make new citizens, and the kenetic boost does a great job of giving them an early military boost to help offset the fact that they're going to start out "breeding" slower than all other life forms, at least starting out. considering how much they really do need that durantium too, i think they should have at least 1 or 2 durantium resources in their starting solar system. otherwise, if they're not lucky, they will only be able to produce 1 citizen every 10 turns, even with several core and colony worlds. this just cannot compete with the other races in terms of early empire growth. I was thinking that there could be different citizen building projects too that could make specialized citizens depends on what special resource was used to create them. like having a citizen building project that uses one promethion instead of durantium, but those citizens will have 1 higher int and 1 lower diligence, or one that uses antimatter instead, then gives 1 extra resolve and 1 less diligence. i think this would be very useful for allowing them to be able to rely on more than just one resource for their entire empire (they might want to use the durantium for other stuff a lot too). It would also make sense that robots would be a little better than other lifeforms at choosing how to build their new citizens to make sure they're going to be well suited at the assigned task they're being created for. and special resources aside, the construction cost is far too high for a starting civ. synthetic already has no growth rate and costs special resources for citizens. making the construction cost that high just makes it unfair. considering that other civs are able to build up their planets while producing citizens means that synthetic will be both producing citizens slower (while using special resources to build them) as well as building their planets up much slower, because so much of their planet production time is put into citizen production. I know that they might have a big snowball population problem later in games after construction is really big on the core worlds and there's no food system to keep them in check, so maybe the citizen building projects could cost a lot less, but have a cooldown of a few turns to help make sure they cant grow out of control too fast.
Onyx: I think that they need a better starting world, more like the yor. nothing with a -50% gowth rate though, with them already being at half growth that would just be horrible. but something that right from the start is adding the resource that they need to grow and thrive, promethion. Considering how much their civ needs it too, it would make sense on a couple levels that there would be some promethion resources in their starting solar system. completely relying on luck of having nearby promethion means that silicon life forms are going to be unable to compete with the other civs most of the time. and even with 30 starting promethion their growth rate is half normal, so it's going to take a lot of time and mining to get them up to normal growth levels i'm guessing. considering the descriptions of the onyx, combined with the fact that they have very slow growth rate and really need starbases early on to get promethion, i think that a good idea might be to have the citizens in their starbases add to the population growth of the nearest core world. i think that this would make sense considering how much they enjoy living off planet, and would make starbases much more prevalent and useful in their civilization that enjoys being off planet on structures that resemble asteroids... like their rocky starbases.