I just scanned those threads that you linked on the previous page (from steam/reddit), and after the opening thread titles questioning whether a 6 character party was too large, there were at least as many if not more posts and upvotes in support of a party >4. In that respect those threads seem to mirror what we've see in this one, and also what Try2Handing mentioned about the intractability of personal preference.
For my part, I'd say that XP scaling is a ready solution to accommodate Parties of 3-5, and also parties of 6 like the earlier BG games. I think it would work well going from 4 to >4, or from 4 down to <3, just as BG1/2. Provided the player intends to play the game with a party. If they don't, then they may have extra requirements to accomodate.
You and mrfuji have both expressed the idea that XP scaling will be insufficient for parties <3, and particularly for solo or duo play. That may very well be true, but it seems to me that if accommodating the Lone Wolf is so much more onerous, then it really should be an independent game mode and not try force a round peg into a square (or ideally hexagonal) hole here.
For example, I should not be able to simply dismiss my party of 4 down to just myself as the Lone PC, and then suddenly gain access to a host of extra abilities, more hitpoints, double concentration or whatever. Unlike going from 3 to 4, or up to >4, where we can keep an internally consistent and flexible method via XP scaling, Lone Wolf likely requires a totally different set up to function well. It should have its own independent game options toggle and should not intrude so much on the default party based game.
Going from 3-4 to 3-5 or 3-6 is relatively simple and straightforward to execute. Solo or Duo play not so much, I think our request here for 2 extra party slots with XP scaling is much easier to pull off in a way that will satisfy both players who like a party of 4, and those who like a party of >4. But going the other direction is like a totally different convo, that requires something a lot more involved to make this thing satisfying for strictly Solo players who aren't interested in a party, and just want some kind of AI henchmen type thing (if they even want that?). I suggested earlier that they could use a ghost chain, or even simply have the camp as a place for story delivery without actually bringing the NPCs along for the ride. To me that would defeat the point, but they could set that up I'd think as a game option, and it wouldn't spoil anything for my preferred style of play. I don't oppose that, I just think it will require more work and a different sort of work to pull that off, than our request to have 6 at the high end rather than just 4. By comparison the 6 request seems pretty straightforward, the 1-2 request rather more involved.