Very cool idea, I love to play with others in this way, together but separated 😄
In the mod description it says "Intended for at least one player on each planet". How playable is it when you're just two, or three?

A good suggestion is to use a doppelganger.

Yes that makes sense. I'm not sure how to realize it, though. What would you suggest?
Looking for it, I only found this mod: https://mods.factorio.com/mod/multiple-characters

Very cool idea, I love to play with others in this way, together but separated 😄
In the mod description it says "Intended for at least one player on each planet". How playable is it when you're just two, or three?

I'm trying to play with some friends now and having trouble getting a consistent team, so I understand the frustration! It also means that testing hasn't really been done much, but I've found at least one known problem (accumulators are locked so Fulgora EM science can't be made. Fixed with today's version 0.1.3). Right now the tech tree gets gnarly and is probably unplayable if you are missing any planets. Doppelganger solution might work, but I don't know how to implement it. If you have some fixes go for it!

What is required to be able to build space platforms with thrusters? Are any planet’s science packs obligatory? Once you're in space and moving, anyone can go to other planets to catch up with the respective research, similar to normal Space Age.

That was my intention with this mod- each player would essentially be responsible for their planet's science. You share scientific progress, but some techs now require your Gleba player to carry his own weight (for example). You'd essentially end up building 4 parallel factories. Once you got to space you could start sharing other things (for example, spreading foundries to the other worlds). That was the intent anyway- my friends bailed on me so I never got to actually play it :(