I think exploring a covenant in AM should be a lot bigger than a simple dungeon run. The bottom level of the cellar is not accessible by any magical means, there are exotic wards in place that the PCs cannot dispel, and they have to find out what this is exactly.
Here's an idea: There are as many sancta as PCs. In order to enter the sanctum of a magus, they have to find out how he circumvented his own traps, maybe acquire some token that he used so he didn't get fried. Once the sancta are opened, they can read each magus' diary. The diaries tell them what the magi were up to and point to lab texts that contain breakthrough points. The PCs have to spend a season in order to translate the lab texts and learn the dead magi's shorthand. The diaries also point to some magic items that are keys to unlocking the cellar and that they have to investigate in the lab in order to find the trigger. (Maybe the lab texts for them are missing or incomplete. I discussed this scenario here) They need to look in the library which books were used last to get more hints. Maybe it contains initiation scripts for some virtue that's necessary and they need to read up on the cult lore first.
The point is: I want them to spend a year in the lab's towers, working to uncover it's secrets. Do their research, come together to share their puzzle pieces and determine the next steps, then get back to the lab for more research. They should get a feeling for who the magi were, re-trace their path, get into their heads to understand what's going on. It will take lots of real time to think about what to do, then fast-forward through the season. Then, as a reward, they can finally enter the cellar, where they discover some bound spirit/dragon/angel, and need to decide whether to release it or keep it imprisoned so they can harvest its power for themselves.
It's not a dungeon crawl, it's a mystical journey, and most of the rewards are earned along the way: breakthroughs, magic items, a new virtue.