Malevolence: The Sword of Ahkranox seems to tick all the boxes, minus the multiple playable characters.

Haven't played it yet, but there appears to be an unlimited supply of quests, and little to no need to follow the main storyline.

It's been several years since I played it, and I never finished it, but have you tried Underrail? It's turn-based, and I remember there being plenty of sidequests. It doesn't have different characters and beginnings though. You create your character at the start. It's very open too, so not very linear.