Governors are easy. Sectors, terraforming, colonisation, habitat construction, megastructure construction. It's only military that's an issue, and assuming you merge generals and admirals (no real reason not to) then that's three out of four leader types covered by one guy half-assedly posting in one thread.Well yeah, we already have that for scientists, anomalies, archaeology, and special projects. They're things that only work if there's a scientist doing them and so having a cap actually matters.
But it's much harder to have a consistent set of those things that works throughout the game for any of the other leader types, and if you don't have that for all of them then you might as well not have the other leader types because the player is not having any interesting interactions with them.
So don't. Just have one new leader type that does Interesting Leader Stuff and automate all the things that are a boring stat stick.
Assign military leaders to manage conquered planets? Remove the empire cap on Really Big Ships and instead require a military leader to crew them? Making leaders a shared pool has exactly the problem you described earlier, where all possible Cool Things need to be globally comparable as opposed to just being comparable within their own lane.