I've been struggling with having fun in the late game as well.
I still think Stellaris has the best RP potential of any game, and personally I enjoy tedious population and economic management. I hate map painters, intensely, so I just RP and build a few planets.
Lately I've been doing very long runs with humans only, in an empty galaxy, slowly expanding and building up sectors (only within their 4-jump range, the rest I leave as empty space, they can contest on their own after I release them later), and releasing them as vassals. This is not great, as vassals are really poorly-made in this game, they can't do anything at all. But I still build them up and keep them as vassals until I colonize the galaxy... then I will release them and see how they interact, make war on each other, make their own federations etc...all under the umbrella of my galactic custodianship.
It takes a lot of time and patience, but this long-term RP storytelling is what's keeping me alive through Stellaris' shortfalls, of which there are many, vassal gameplay being foremost.
I truly hope they will do something more elaborate with feudal empire soon. I think it would make business sense too, with Dune being as po[pular as it is, it might bring in new players.
Tbh they should act fast and make it their next DLC theme: a vast galactic feudal empire, with LOTS to do internally. Dune will sell this for Paradox. There's also the Foundation series, but that train already left the station. Still, it can be done, in this year, and piggy-back on the current sci-fi trend