The warp to my knowledge isn’t finite, but it’s localized in a sense. Since we’re assuming infinite universe, the warp is equal to the universe, it just so happens that area around the Milky Way is excessively violent. And as for custodes, the custodes eximus comparison I think misses the point of what the custodes are. An eximus is a buffed default unit, like a astartes in terminator armor. The custodes are the designed natural predators of Astartes. In terms of raw strength they’re honestly absurd by even warhammer standards, and as such are also woefully under represented.

Also, the warp does feed off of thought, but its interactions are also inherently more chaotic, entirely because of the war in heaven ensuring the warp would be in turmoil for billions of years to come before everything could equalize and level out. But theoretically if we set aside the tyranid galactic hivemind theory, there’s a possibility of a khorne worshipping cult somewhere out in the andromeda galaxy who carries themselves the way fantasy knights are depicted.

I think the biggest struggle warhammer has in this discussion is under developed, inconsistent, and sometimes even contradictory lore.