You can design your defensive bases that are built upon the star base template. I suggest building mediums as the light ones aren't that powerful.

Yeah, also reactivating the research on Hull Plating helped, but still a bummer, that Stations don't have a shield :/

If this is in the very early game it's possible that you don't have sufficiently advanced reactor tech for stabases? It's separate to the warp drive tech. I think the starbase design will always be limited to components at or below the tech level of its reactor.

Not sure about early game/ reactors, but I had the chance to research another/better Plating Tech pretty early on. Since Starbases don't switch to it, I was stuck with 'MK I' until I re-allowed it and teched-up. Still stuck with Polarized Plating though.
Honestly I don't have a clue how most of this works (from a modders viewpoint I mean), but since you apparently have a lot of experience with adding ship design possiblities, I wondered if Starbase Design was also possible (can't design them in Vanilla too, but there it's far less of a problem since it sadly has only a few possiblities for... well... everything kinda).
Not a complaint in your direction by the way, just something that would improve UX.

An interesting approach...if starbases can be assembled like defense platforms according to requirements. Of course you can now influence this with modules, but it would be cool to detail it even more with sub-modules