Temporal Sanctum is a cool concept, but lacks difficulty scaling.
So, my simple idea would be to incorporate it in Monolith, as an additional way to access it (normal access can stay as is).
If you encounter special monolith node, you can enter the Sanctum from there (using key) - and then corruption level and all current echo modifiers apply to the dungeon as well.
This would make echo farming more interesting, while at the same time making dungeons more challenging and rewarding.
But where is floor 80 of the dungeon when you beat the boss and the void starts to break through the walls and you need to race back up to floor 1 before your character is deleted?