We need to get rid of all (full)spellcasting classes.
Have one spell list available to all.
One class.
spells know on level of sorcerer.
Make sub classes with bonus spells: 2 spells from cantrips to 5th level, per spell level.
Have bonus class features give flavor/mechanics to the mage/spellcaster.
then you can have your, battlemage, healer, summoner, greenseer, mindbender,
if you have all spells available to all, then you only need to balance out sub class features, as players will soon pick out most useful spells and you can count on that in balance equation.
And since it's based on spells known, spellcaster cant be swiss army knife and have new sets of spells after every rest.
Just replacing one spell every level.