I think what I'd do in Larian's shoes, if the plan was to increase party cap up to a maximum of six, would be a two-fold effort broken into a few steps:
Rebalance NPC stats to mirror 5E monsters list to the letter
Slight homebrew by making their attacks a slight bit stronger (make them individually more dangerous) - Because I believe the current reasoning for the bloated HP pools etc, is because of not wanting too many enemies in a particular battlefield because of turn-based turning into a slog if too many units active at once. Consequentially, this would likely make each encounter shorter if played well, and appeal to the portion of the playerbase who already finds it too tedious, or would argue that a bigger player party would require more enemies to balance out, because they'd individually die faster.
Still balance the game around a party of four.
Repurpose the difficulty system from Divinity: Original Sin 2 into a balancing measure, where if the party has more than four members, increase the enemy stats by 10% and have the [Hardcore] tag on them, which in DOS2 means those enemies would only appear in higher difficulties. So yes, bigger player party would mean more enemies, but combat would be about just as long and "tedious" as it is now, except for more enemies but generally dies faster / fewer turns.
It's been a thought in the back of my mind, and I think that's the solution I'd settle with personally.