I managed to create a wizard who knew all of their possible spells at level 7. They couldn't level up to level 8 because I didn't have any spells to select. There is, of course, a workaround: hand a spellbook to a party member, then level up. This forces you to double scribe two spells, but it is better than blocking the level up. Ideally, this workaround should not be necessary.
Also reference this thread: https://forums.solasta-game.com/forum/the-know-it-all-creating-a-mage-with-as-many-spells-known-as-possible