Unlimited 10th level spells: Arcana Cleric + Boon of High Magic + Simulacrum + Wish + Book of Exalted Deeds
This combo allows a wizard to have access to an arbitrary number of 9th level spell slots. This answer by Ladifas outlines the basics of the method. In summary:
- Cast simulacrum on self
- Command new simulacrum to use wish to cast simulacrum on the original caster.
- Repeat step 2.
Since we have the Boon of High Magic, we have two 9th level spell slots, so every simulacrum created would have one remaining after using the first to cast simulacrum. Since the simulacra follow our commands (or can be instructed to do so) we have access to an arbitrary number of 9th level spells at will. It takes some setup, but the up front cost is only a 7th level spells slot and the components for simulacrum.
The above method works for unlimited 9th level spells for any character with access to both wish and simulacrum.
However, an Arcana Domain cleric can use the Book of Exalted Deeds to make those 9th level spells into 10th level spells.
The Arcana Domain cleric can learn wish and simulacrum using their Arcane Mastery feature:
At 17th level, you choose four spells from the wizard spell list, one from each of the following levels: 6th, 7th, 8th, and 9th. You add them to your list of domain spells. Like your other domain spells, they are always prepared and count as cleric spells for you.
Then, when we read the Book of Exalted Deeds, all of our 9th level cleric spells are upcast to 10th level:
Once you’ve read and studied the book, any spell slot you expend to cast a cleric or paladin spell counts as a spell slot of one level higher.
Simulacrum duplicates all of our statistics:
the illusion uses all the statistics of the creature it duplicates, except that it is a construct.
The Enlightened Magic feature is copied when we cast simulacrum.