Propably the reason party control is done in such an annoying way is the console and playing via controler there. I see no easy way to make the old (and way better ) controls handled by the controlers. At least there should be a possibility for the PC version to work as the way more comfortable party control as in most other games.
I don't think planning for controller support here is the issue. Recent games like Pillars of Eternity and Pathfinder Kingmaker have controller support and they do not have the issues that BG3 does. Diablo 3 on consoles works even better than mouse and keyboard, IMO. Plus the enhanced editions of the Infinity Engine games added controller support on console and they play wonderfully (even the touch screen support on tablets is pretty good).
I think it's a Larian problem where they created a control scheme that simply does not work as well as other established CRPGs and they're sticking to it just because it's the system they created. They need to learn to "kill their darlings" for the good of the game.
I don't think any game designers good enough to make CRPGs as loved as DoS1 and 2 are would ever look at those games' weird jank-*** controls and think, "Yes, this is a better solution than the system almost every CRPG has used for the last 20 years. We've reinvented the wheel."
I think it has to be a technical limitation of their engine that only one character can ever be selected at a time, and Larian designed the controls around it. If that IS the case, all I've got to say is that Larian told us BG3 was a AAA game, with a AAA budget, and they hired up to build it. A AAA game dev would update their tech so it didn't cause problems that could only be solved with bad game design.