To me, an effective ambush means everyone attacks at the same time, after which everyone should then properly be added to the initiative order.
It can be handled, if one feels it is needed - Solasta has a surprise round - enemies who were ambushed don’t get to act on their first round (works well, there, might be too much for BG3 as characters can move rather far in one turn). There could also be a free half-turn for ambushes - like in Pathfinder games. Possibilities are many.
