You procgen the base planet and tune it manually after. Something similar to what Bethesda is doing with Starfield.
Everybody know it, CIG has always said that every procgen creation will have a human pass.
I know they've said that.
But what we are talking about in practical terms is CIG deciding what will be in every system, and on every planet. Do you think the system is generated based on rules? No, each planet will be decided upon by CIG, this system will have say 1 gas giant of x size, 1 icy planet of y size, 2 moon of z etc and they will design each system. No, wait, this system should have 2 gas giants and x number of moons, etc.
They will design exactly what will go in to a system, on to a planet, etc. Now they will use tools to 'generate' the planet, sure, you could call that procgen if you like.
They are to all intents and purposes manually creating every planet, every POI, every feature, every river, every settlement, every identifiable asset that you think differentiates SC from other games and being immersive and detailed.
If CIG want to produce 100 systems with raw planets and randomly placed POI like in ED, they have all the tech to do it with few clicks, but that's not the plan.
They will never be able to create 100 systems with planets and biomes though. Because they are doing them manually. Tools like painting the surface block by block with detail and features takes a long time.
What are Bethesda doing? I'm not sure. I suspect the vast majority of their systems and planets are generated as they are in ED and NMS, and then they are manually adding all the core storyline cities and outposts and missions as they would with Skyrim, just being spread out over many systems and planets rather than over a single undulating landscape map. I imagine Starfield are doing something mid way between NMS (basically all procgen) and SC (basically hand crafted, hence 1 system so far).