Using "Sync map gen settings" to disable lab tiles and base the stages off an actual seed doesn't work usefully with Space Age: the resulting map is a mix of all planets' features.

Would it be possible to have a way for the stage surfaces to be based on a specific planet?

That would be what Sync map gen settings is supposed to do.
Marking this as a bug.

Turns out, every planet has their own map gen settings "derived" from what you input in the world creation window, and there's a bunch of hidden map gen settings too.
As a solution, I've added a feature v2.3.0: you can now set the map gen settings to match a specific seed/planet.

This doesn't quite work; when calculating the seed your TS code does:

if (seed) mapGenSettings.seed = (seed + (planet.map_seed_offset ?? 0)) % 2 ^ 32

but unlike Lua, ^ is bitwise XOR, not exponentiation, and has a lower precedence than %, so this ends up adding the seed offset, then reducing it modulo 2, then XORing it with 32, and so the resulting seed value is always either 32 or 33. :)

oops. Hopefully Fixed in next patch.

...Kinda hard to test, and I'm too lazy to refactor it to add a test... uh... should be fine, right?

Seems like it works now to me :) Thanks!