How to upscale the death house and still keep it fun?

Due to Reasons™, my intro level to my current run of Curse of Strahd for a new group didn’t involve the Death House, I wrote my own intro-level this time.

I always had a lot of fun running it for other groups, though, so wanted to keep it in the game for a later stage. I’ve already written it in the world in a place that’s pretty likely to be found by the players.

The issue is that I still want to keep it a Death House, which it clearly is at levels 1-3, but I expect the party to find it at the earliest by level 5, probably 6. While it still would not be a breeze, I ran a number of the encounters with likely upscales of their current party and it did not feel deadly at all. I could just up all DCs and make each monster group a bit larger, but I don’t feel like that’s particularly interesting.

Assume my Plan™ survives first contact with the players and they find the House. What is an interesting way to upscale the Death House to party level 5-6? The party will be either 4 or 5 characters, depending on who can make which session, and the group is paranoid enough about dying that they seem to insist on running at least 2 healers at all times.