My compromise if the premise is involving all 4 would be to give you and Hop Zacian and Zamazenta as your Pokemon rather than CPU allies, maybe as a "Victory lap" after winning a regular battle raid with Eternatus. This also contingent on Marnie being a character and not just there for rival quota.
Having plot Legendaries under trainer control is something I wish the series dabbled in more, only really doing it with Gen 5's Dragons, then Gen 9 with the Raidons and Kieran briefly capturning Terapagos. I think selecting you two as the trainers is a suitable enough stand-in for the role of the two kings, much like the mentioned battle with N between Zekrom and Reshiram added to the party in Gen 5 Part 1.
Yeah this was exactly my idea. They could've done both! Why not!
I mean I even mentioned that was an option I just don't think it's that great an idea for the situation and would exist specifically and only to facilitate a team up with your rivals that is purposely not set up because you just fundamentally do not have that relationship with those characters so we can have a 4 character raid.