Unfortunately you do have to use the premade characters if you need teammates, but perhaps that will change as Larian does more work and gets more feedback. Or perhaps not. There is one exception, that is if you start a multiplayer game, each player creates one custom character each, then some of those players disconnect.
Baldurs Gate 2: Enhanced Edition you can actually fill your party with custom characters which is nice. I think there is a lot Larian can learn from having a look at games like NWN and BG:2, things like the interface etc (like I wish there was WASD key movement like in NWN).
And spot on with the early access stuff. I largely got it for the opportunity to play and build with DnD 5e rules, and was surprised when I got a lot more, some of the graphical stuff and tactical stuff was incredible. For most people I would recommend waiting for the final product, but for diehard fans it might be worth it.
There hasn’t been an update for a few months now so I’ve stopped playing this for a while, I assume Larian is very busy working to refine the product further for a big release to revive interest and get more feedback. I’d probably give it another run when there is additional content like extra classes or extra areas to explore.