Rather than a megadungeon, I'll propose shorter, adventure-themed dungeons:
- Forge of Fury. You said you've already played through Tales from the Yawning Portal, so maybe this is already off the list? Regardless, it's a challenging and well-executed dungeon crawl for 3rd or 4th level characters.
- Pharoah. The1E classic set in a pyramid. This will definitely challenge player skill as opposed to character skill.
- Thunderspire Labyrinth. A highlight from 4E. There are a few factions, but nothing overwhelming. The conversion from 4E might be slightly challenging, but it's mostly a matter of dropping the AC and attack bonuses.
- Storm King's Thunder. I see you consider Against the Giants to be a slog. Having played through SKT, I don't think it's a slog. There are 5 giant lairs that are each uniquely evocative and take about 2 to 4 session to complete. A solid option for Tier 2.