GTA5 is probably being done by the same team that did LA Noire port, Virtuos Games. At least to me, that make the most sense for Rockstar to continue working with the same team.
Virtuos are pretty busy at the moment, they're doing the Switch versions of both Dark Souls Remastered and Starlink. You're right though, I would also expect that if/when GTA 5 gets ported to Switch (and come on, it really should, there is no good reason at all not to bring it over) Virtuos are the logical choice.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023.