What they really needed to, and what 13th Age did, was to basically have lower level spell slots disappear as you level up.
So it would be something like this
Level 3: 4 2
Level 4: 4 3
Level 5: 3 3 2
It really doesn't matter if the number of lower level slots goes down (and eventually level 1 slots disappear altogether), so long as the overall number of slots goes up. You can always use a higher level slot to cast a lower level spell, but this way you keep the increase in actual slots on a more controllable curve, even as the power level of the spells increases. (And also looking at the above, it isn't necessary to go straight to 2 level 3 slots if the PCs are fighting less encounters between long rests).
Cantrips count for a lot, and you could give higher level options to master low level utility spells at a certain point so that they can be cast at-will (like Warlock invocations).