XP split doesnt really work for the endgame when the encounters are balanced around a full party at a max lvl.
"Balanced for". It always worked in the past for plenty of other titles, not sure why it shouldn't here.
And adding more or stronger enemies sounds easier than it is done. It will require to balance encounters for every difficulty level for different party sizes
No, it wouldn't.
considering that we are talking about enemies that are smarter than in other DnD RPGs
irrelevant. The AI manages whatever enemy is set on the encounter. It doesn't require to script individual moves for each enemy. That's the very reason Larian worked on a "proper AI system" since DOS 2 and something they advertised strongly in the past. You'd literally just need to swap models in the toolset to already have it somewhat working, with occasional minor adjustments required.
n BG2 for example you dont have enemies wake up their sleeping allies or throwing healing potions at them.
That doesn't really change anything in practical terms. If you need to tone down an AI to make an encounter viable, maybe that encounter needs revisions to begin with.
Also bigger fights with many enemies and big parties will become a slogfest especially if Larian will add proper reactions. There is a reason why Solasta went with a 4-person party too, imagine how many pop-ups for Bend Luck you will get in a turn in a fight that has more than 12 people?
No, it's not. Which is precisely why Solasta gives you several occasions to escort one or two extra NPCs for long stretches of the game.
That said, we are still blabbing as if some utopic "perfect balance" would necessarily need to be hit to make a game of this type work, which is a bullshit premise to begin with. A competent designer needs to just to account for extremes (make sure they are still somewhat viable) and choose a suitable middle ground as the standard reference, not lose his mind after the "balance" of every single variable.