I am firmly against the shared xp model for games. First of all, that model means that unless the number of companions remain relatively limited and things are designed very carefully, there will be at least a few companions that end up falling behind in levels, which makes for less flexibility in party composition later down the line, plus and more importantly in my opinion, it makes it harder to fully enjoy your favourite character because you have to swap them out to both keep a balanced party and keep everyone leveled equally.
Second, I am of the opinion that the abilities granted at the maximum level a game provides is part of the promise of the game. That by the end you'll be able to enjoy all the power and tools available to you to some degree or another. And I think it's important that any player who plays the game to completion, whether they be casual or super into systems mastery, should be able to enjoy that content. Therefore you shouldn't incentivice playing with fewer characters in order to reach that max level. In Pathfinder: Kingmaker I played through the whole game with a full party at all times and only made it to level 18 or so, which was a big bummer. I think all characters should earn xp at the same time and that for an RPG, you should be able to enjoy that maximum level for a decent amount of time, at least the last 10% of the game, maybe even the last 15%.