I'll address a few points.
There are mods to go past lvl12 but you'll need an xp buff mod to get to 20 which is the normal max for 5e.
While the protagonist of BG1/2 plays no role,
Bhaal is very involved in the proceedings of BG3.
BG3 is 100+ years after BG2 so anyone not of a long-lived race is almost certainly dead. Minsc only survived because the big goof got turned to stone and spent most of it as a statue. He and Jaheira are both potential companions.
