Hmm, I played around abit, I don't think terraforming starbases are gonna be a thing.
Putting a trigger on starbase module construction was easy and worked.
When I switched around the effects it turned out to be more complicated.
I used the 'ConvertDeadPlanet' and 'ConvertDeadPlanetProportional' actions. The former didn't seem to work at all for me, the latter stopped working after I introduced a scope to the trigger. I had similar results for 'ConvertDeadPlanetToThulium' and 'ConvertGasGiantToPromethium'.
So I apparently could make a module which triggers a mega event (uses the ...proportional actions) but not one which does so limited to the ZOC (or AreaEffect) of a starbase... I could try 'DiscoverNearbyPlanetOfClass' (which is used by the benevolent traits)... but I suspect this also breaks down with scopes.
Another weird thing I encountered was that you apparently can't make 'Planet' a prerequisit target for starbase modules... or I did something wrong.
I presume in the internal code these functions were made for one purpose only and don't support scope, target etc.
At least I got to see a 4-promethium deposit in the process and a thulium class 11 world (the two models have different height offsets) ^^
Culture flipping should be easy enough to do this way though since there is a merc component which flips (non-capital) on starbase construction.