The Star Trek New Horizons mod should provide what you are looking for, although it's been quite a while since you last played it. I'd be much better off just conquering or subjugating my allies, and that sort of betrays the point. Which is one of the reasons why, despite really wanting to play the friendly militarist xenophile egalitarians and form great Federation, every attempt ends in regret. I don't even expect them to help me, but once your empire has surpassed a certain size, gateways become crucial to projecting your power in an area. And I don't trust my allies to actually fortify important systems or place gateways so my fleets can go help them if they need it. Plus, you need to deal with your allies declaring wars they can't win, wars they won't actually fight, or just having war declared on them (which means only that particular ally can send a peace offer, but of course the entire Federation has it's diplomatic options limited, whether it's related to the Federation or not). Federations are still not a viable means of expansion, certainly not compared to warfare.

Unfortunately, this is something I think the Federations DLC completely failed to adress.

How to play stellaris correctly