It’s in the FAQ pinned here I the discussions. Tried to copy and paste it but my phone doesn’t want to.

From the FAQ:

Nemesis: The Nemesis DLC adds a few additional base game espionage operations to the game
Utopia: Advanced slavery options added, included Remans added as battle thralls, unique slavery options for the Orion, Breen
Synthetic Dawn: Two new races added to the game, the Cravic and the Pralor become playable
MegaCorp: The Orion & Ferengi are converted to megacorp empires with all the unique associated features.
Apocalypse: Planet Killer ships added to the game for the Undine, Scimitar, Xindi & Krenim
Federations: Unlocks the federation types included in this DLC. Note this doesn't impact the formation of the UFP.
Aquatics: The Chelarians have been added as a playable race and the Selkie are includes as a warp capable primitive

Thanks for that, completely forgot to come back and paste them when I got home. To add, I THINK the new vassal systems work with the Klingon Houses, or at least helped in redoing them.

Are other DLC counterproductive to the way STNH wants to run? Like adding features just for stellaris' sake may not be meant to work well with STNH if the mod wasn't specifically utilizing it

No, because if the team doesn't want a feature from whatever DLC then they'd just not include it. It's a total overhaul, so only what they want is used and the rest is overwritten.

another option, try the Star Trek New Civilizations mod as well. It was the same mod for a time, and then part of the team wanted to do it a bit different. I find it to be more stable, but similar in feel.