To long didn't read: Just make amount of characters in party up to 6 -ish. Make it an option. Best solution based on your discussion above after me. The "perfect" balance doesn't need to exist. It didn't exist in BG2 or DOS2 they both sold well.

Did read:

Since the party size discussion drifted into difficulty levels just want to point out one thing. You mentioned above on page #70 Pathfinder tends to get borderline easy from midgame. I played BG2 & 1 + TOB + Pillars of eternity. They all tend to have that characteristic in my opinion.

In case of BG2 especially since levels 12-14 are a major breaking point for some classes(mages especially) in 2nd edition DND. Especially with spell sequencers coming into play.
The game was easier for me with a party of 5/4 than a party of 6 because you would reach that braking point faster because of experience split. So experience split will normally act as a " balance" factor. You will take longer to reach the breaking point.
I still played with 6 in majority of my playthroughs for party interactions.

In PoE depending on the builds you make it's the same thing. Once you get all the spells you needed to make your " perfect party" becomes perfect. It can face all challenges because you designed your party around it.

Is becoming OP an issue? I don't really think so. Heroic fantasy is pretty much about it isn't it. Knowing where their system would lead bioware mae BG2 a story about becoming a living god and most of those who played it liked it.
DOS 2 did the same and thank god cause you can two-tap the endgame boss with the proper party(viable only for those of us who make multiple playthroughs and learn in-depth mechanics. Most of the people playing a game don't do that, they don't even finish the game!!).

While party size impacts gameplay it doesn't mean the consequences it leads to are necessarily better or worse. It's just different. Let people play the way they like. Both aspects have their pros& cons and they are all listed in one of the 70 pages of discussion.

Alt+ left click in the inventory on an item while the camp stash is opened transfers the item there. Make it a reality.