UngeheuerLich
Legend
Simply cap spells known to 4th level or less (maybe 5th level or less). Casters can keep the slots.
Thoughts?
Possibly create magical items that allow casters access to higher level spells for atunement and sufficient class level.
Probably you can cap it to level 4 and below, but treat spells of higher levels as magic items. Maybe even attach them to powerful items.
So a fighter gets their +3 swords of sharpness while the caster gets a staff that allows them to cast certain higher level spells 1 time per day/short rest each using their available slots.
I like this, because both fighter and wizards can use their attacks/higher level slots but not at full capacity when not having access to their tools.
I might try actually try that approach. It really balances out casters vs martials by making both equally DM dependent.