OK, I have a few more queries about the set up.
1. Is teleportation blocked both into and out of the sanctum? Is it blocked within? I assume the answer to the latter is no given Sinuhoteps Space Rending Spell ability.
2. The door is pretty impenetrable but what about the surrounding walls? Adamantine weapons are pretty ubiquitous at this level.
3. Are divination effects impacted at all? Gloves of Reconnaissance are very common at this level which may spoil quite a bit of the surprise.
4. Likewise, using summoned elementals to earth glide through the walls to scout is a common tactic although I imagine Sinuhotep would deal with them rather easily.
Lastly I just want to say thanks for a very interesting end boss. I rather pity any party that chooses to fight him, especially if run to his full potential. Surma also looks like a great fight. Choosing summons effectively immune to her gaze attack made me chuckle.