So long as the difficulty is exactly the same, which I'm afraid won't be possible. And that it's a hard toggle, and no way to recruit past 4 without it on.
Because if you can, you have to. That's just how it works sometimes.
But why is this Larian's problem? Isn't this your problem? You are choosing to make the game easier for you by taking six, in exactly the same way as those who are choosing to make the game harder for them by taking only one. You cannot legitimately complain about the game becoming easier by saying you are somehow "compelled" to take six. No one from Larian showed up at your home, put a gun to your head, and made you do it.
This is similar to the "don't like it? don't use it" argument that's also used for other Larian homebrew like lack of long rest restrictions, eating food to heal, shove instakills, bonus action jump+disengage, wizards learning cleric scrolls, trivial stealing from merchants, etc. (Note: I restricted these examples to things that are actually possible to avoid, unlike e.g., BA Shove which enemies will use against you.)
This is Larian's problem because games require restrictions; it's not fun to have to purposefully nerf yourself as a player. And Larian allowing a party size of up to 6 by default (meaning: the 5th+6th companion you find will automatically join your party if asked, and the game wont warn you that "BG3 is balanced for parties of 4") is poor game design. Players will have to consciously decide to continue to play with 6 or play with less than a full party, either making the game easy or having a game with less party banter & companion-specific-story-interactions respectively.
My proposed solution to the above is a hard toggle in settings that defaults to off, that isn't advertised, and that comes with a warning "this game is balanced for 4-person-parties." This would be enough of a disincentive that I wouldn't feel bad playing with 4, but would still allow people to play with 6 if desired. Also add a "split exp" toggle, so your party-of-6 can either be balanced or not: the player can choose.
I never said anything contrary to Larian issuing a warning to players who use the option. In fact, I have said it myself in previous posts that Larian can provide a very stark and clear warning, for example a large popup that comes up on your screen the moment you toggle that option.