Class conversation tags doesn't really seem like a thing that Larian would worry about. But if they are, then its simple enough to restrict the tag to the class chosen at character creation, or ask the player to chose which tag to use when they level into a new class.
The fact that some builds are bad is also a terrible reason to nix multiclassing. Let people try and fail! We don't need guard rails around every single experience! Give a respec option, or let people restart like an old school video game player.
Besides, 5e is very forgiving of builds. Its not like we are playing Wrath of the Righteous here.
