I'm guessing P5R will be the experimental project for the "new" P-Studio. If they wanted to experiment before Persona 6 it would make sense to do so around a game that's more likely to be a traditional Persona experience, that way they can make changes to the series formula and gauge the response to those changes.
Credible sources (Jason Schreier) have strongly hinted that Persona 5 S is not an RPG. ATLUS has done a similar thing in the past, between P4 and P5, with the development of Catherine. Catherine has been said in interviews to be an experimental, growing experience for the team to challenge them in a genre they're not comfortable with. It makes sense for P5S to be an action game developed by P Studio to test the waters for what they potentially want to do with Persona 6.
I don't see how an enhanced remake of an old game has more potential to be more fertile grounds for experimentation than a new game in a completely different genre.