Perhaps the goal could be to build the new FTL interstellar Spaceship at the Edge of the solar System? So you have to ship those expensive parts to there?
and the research costs should be way higher I guess?

That sounds interesting, I hope Feoras has the time to make an Space Age variant.

FTL parts could instead just be needed to be shipped to ship in orbit and assembled into FTL drive on the ship. You could make it so the FTL drive is necessary to travel from solar system edge to solar system edge, from one star system to another like: https://mods.factorio.com/mod/Starmap_Nexuz.

Or it could be kept simple, using this as a basis: https://mods.factorio.com/mod/Space-Age-Without-Platforms

It would be cool to get this working together with space age, however many items that were previously used got removed (e.g. the satellite).
Nevertheless, it would require a complete new balancing of which items to use to craft the components and how much/which science packs to use ...
And sadly I do not have the time needed to make this compatible with space age currently.

If someone would be interested to work on such a compatibility, please feel free to reach out