Yaarel
🇮🇱 🇺🇦 He-Mage
Your suggestion for an "At Lower Levels" mechanic is excellent.If I were to "downlevel" Fireball, I would have it be something like this:
That would make it so 1st level fireball would be a 10-foot radius sphere that deals 2d6 damage and 2nd level fireball has a radius of 15 feet and deals 5d6. That's still pretty powerful for those levels, but not super game-breaking.
I can see uses for this, where it is appropriate to get a spell at a higher tier, but once having it, can use less resources for it.
For the Fireball spell specifically, I would rather correct its damage to balance alongside other spells at slot 3. Having weird damage for Fireball makes it more difficult to balance the other spells, because Fireball warps the comparisons when deciding how much damage each slot should deal.
Then if designers want the Wizard to be iconic with Fireball, then it needs to be a Wizard class feature (or subclass feature), to add damage to the spell.