Wow, talk about being in harmony. The Secret of Bone Hill was one of the first adventures I ever sampled (albeit with two total players: a DM and myself). And there is the follow-up, The Assassin's Knot, more of a mystery + RP.
As for systems, I prefer AD&D/OSRIC for a few reasons I don't need to detail. OK, I'll share one. Damage output is higher for non-casters which improves the pace of combat.
Megadungeons. Gillespie’s dungeons would be major, if not possibly life-long, undertakings. I'm in a well-run Stonehell and Arden Vul game. Personally, I've wanted to play Tegel Manor (awkward to run PBP).
Forbidden Caverns seems to get some fair criticism here:
https://tenfootpole.org/ironspike/?p=3881
I think you could overcome the lackluster writing, but it might be too much work in the end. Barrowmaze seems to have had a more positive reception.