So, slow down casters (not just wizards, after all) a bit and speed up martials?
On the caster side, the fix is pretty simple (at least for a starting point), would be to slow down spell level progression and spell slots as well.
A lot can be done for martials, as there have been numerous threads on this, but no one ever really seems interested in making changes instead of just talking about it.
That would require rewriting the game, to recalibrate everything, but I am ok with that delay in the spell-slot-level schedule access in principle.
I probably prefer a more symmetric progress.
While we are rethinking spell levels, I strongly want to stop referring to "spell levels" and just refer to class levels. So a 13 level Wizard can cast a 13 level spell.
In any case, most of the spells of spell slot levels of 6 thru 8 suck. There are a couple of good ones that always get picked, and the rest are nonchoices, that need to reduce in level or get a rewrite to make them more competitive alongside other spells.
I would be ok if:
• A level 1 Wizard can only cast cantrips. 2 or 3 cantrips are at-will, but the Wizard can cast other cantrips using slots.
• A level 2 Wizard can cast what is currently a slot 1 spell.
• Level 4 accesses slot 2; level 6 slot 3; level 8 slot 4; level 10 slot 5; level 12 slot 6; level 14 slot 7; level 16 slot 8; level 18 slot 9
• except Wish is a slot 10 spell at level 20
Meanwhile, the low level Wizard is tough and survivable, maybe even Athletic.
The Wizard only relies on spells and never gains weapon proficiencies (albeit one school might), and normally multiclasses for it.