Put it in simpler terms: Take a spell caster like cleric wizard and/or druid, give it some number of me dls like 12 and compare how your spell prep changes if you can and can't upcast to higher level slots. Don't forget that sorcerer and wizard are limited to knowing/preparing spells they chose while leveling up& can't just prepare from class list like cleric/druid.
My proposal would be to ditch upcasting completely, meaning that while your Fireballs might get a bit more bang for the buck at higher levels you wouldn't be able to cast nearly as many of them: you'd be hard-limited by the number of 3rd-level slots you have.
I pondered a fantasy heartbreaker that's more or less work like that.
Fear, Fireball, and Fly are 3rd rank spells and cost a 3rd rank slot. Every Fireball you shot is one less Fear or Fly. You can't upcast spontaneously.
An Evoker Wizard or Red Dragon Sorcerer can prepare Fireball 4th Rank to use Fireball as a 4th rank spell. But this requires a preparation. And you only get your Casting stat -10 preparations. Upcasting is a limited class feature.
If you are a wizard of 18 Intelligence, preparing
Fear
Fireball
Fireball 4th
Fly
uses up 1/2 your preparations.
Magic items, such as magic staffs, let you cast spells that you don't have prepared via spell slots, spell strain, or spell points. And that's why you go out your way to find them. And this is why most casters don't carry their attack spells on themselves but do via their magic items. And when you go against enemy casters like goblin hexers, evil cultists, and imperial warmages, the best strategy is to
disarm them. AKA let the martials rush or shoot them.
It's also makes targeting every save very difficult because targeting every save loses up more of your preparations and sucks up your ability to do non-offensive magic.
So spell casters offensively either go all in versus one save OR keep a wide array of targets saves and weaked spread of abilities.