The plan: To establish a commonwealth so early as to essentially have two empires developing in parallel.
The result: Didn't work
I needed to get a constructor and a colony ship through a wormhole with a surveyor so they could establish themselves somewhere else so they wouldn't compete with me for planets. It took about 50 turns to get the technology to fit all three in a fleet and build them. Sent them through and landed on a tile with four resources nearby! Also soon found a planet and several others for expansion but they weren't isolated enough and eventually came under attack. I made them peaceful and diplomatic but also prioritised ship building and fortification, hoping they'd stay out of trouble that way. I probably should have made them expansionist so they'd grab those planets. I jumped in the war but they were dead in weeks.
What I think this says about commonwealths: Their stated purpose to to allow you to split off the less important part of your empire to run itself. It can do that but unless you are twice the military strength of the highest AI empire and give your commonwealth half your ships, I don't think they'll survive. Or they have to be so isolated that the AI can't even reach them. And with bug reports of the AI traveling outside, their range that might not be possible.
What can be done: I think a key problem might be the --- ripe for conquest diplomacy modifier. It ensures other AI empires with start to hate commonwealths as soon as they come into existence. I suggest exempting commonwealths from "ripe for conquest" if their parent civ doesn't also suffer from it. There are plenty of other ways a commonwealth can get themselves into a war so it's not immunity.
Some have suggested having commonwealths share diplomacy with the parent civ, but I like that they wage war on their own. However as it is now, giving planets to a commonwealth is really just setting them up to be distributed among other empires according to who can get transports there first.