Normally in Hex Casting, stronghold libraries have the highest chance of containing scrolls for the higher tier spells, however they can't be found in Better Strongholds strongholds. This is because Better Strongholds uses its own loot tables in place of the stronghold library rooms, and Hex Casting only injects to the vanilla stronghold library loot table. Simple fix would be to add other loot tables for these scrolls to spawn in betterstrongholds:chests/grand_library and betterstrongholds:chests/library_md (can be done in hex casting server config)