I've played with a six member party with a mod. It's not cumbersome at all. It does obviously trivialize the current encounters even more though. That means, Larian would have to rebalance everything again.
It won't happen. The game's bones/core is essentially done at this point. Larian wouldn't market Early Access where core game features can be altered. They have built this game for a party of four and that's what we are getting.
OR, since this is Act 1 it can be allowed to be a bit easier and they only need to balance content beyond Act 1? And I think the point of this EA is so they can modify core features, they have been trying to fix the RNG system for example which is a core game feature.
Possible but I highly doubt it. I have a feeling all the Acts are pretty much done now and are being tested internally. The good news is, since it's still internal, yes there is some chance Larian can modify it to accommodate six members. But it's not just turn based combat. It's also world design. Frankly, I'd be pretty surprised if a lot of their assets haven't been built yet.
I said it's not cumbersome to fight with six. But with the world being so small and compact, I can see it being an issue further on. If the environments have already been built with four members in mind, like Act 1, things can get really tight and crowded adding more and more bodies.