I don't think it violates the DM's oath to run scenarios that are below the party's level in terms of CR - but give them appropriate XP, so they level up slower. I'd suggest running a scenario that's 2 levels below the parties, but do it without fudging; play the NPCs as sneaky. Hopefully that won't TPK the party but it might make them think twice if PCs start dropping. If they then alter their tactics you can increase the threat level (and XP).