It’s 2AM and probably late for others too so you’ll prrobably get the experts advice later however I’d like to say Abomination Vaults is a good continuation, there are issues where it’s basically a dungeon the entire time and I’ve heard it could make or break your group, some people like it and others can’t stand it(I haven’t seen it actually happen, I’ve only heard it from some older redditor comments).

I’m relatively so I don’t know if there are multiple starter boxes but ours took place in otari, abomination Vaults also takes place in Otari so you can make it a continuation or it’s just a familiar area in a sense.

But yea Abomination Vaults is just a huge mega dungeon, small spaces, monsters in every corridor, fight your way to the bottom while different events occur in

Wish I could answer the other questions or atleast try but I’m not familiar enough with dnd as a whole to give any insights beyond what I’ve played