To fix the problem of "Great Spell gacha", I'd like to propose a set of tied changes:

• Defer rolling of what Great Spell an ancient scroll is until loot is generated(it's picked up); perhaps we can use the archaeology mechanic here.
• Once a scroll is rolled, give a hidden advancement or other similar flag that:
  • Prevents future rolls from picking this scroll (until all scrolls are found, perhaps);
  • Adds the draw pattern of the scroll to the Hex Notebook, so it becomes 'known' knowledge.

How much effort would this be to implement?

honestly if I were to implement this (lazily might I add)
I would take a page out of scriptor magicae's book
make a loot item that has to be right-clicked to "identify" it which then results in the unique item
(scriptor uses this for spellbook of all tiers, and all tiers of spell components)