Simply cap spells known to 4th level or less (maybe 5th level or less). Casters can keep the slots.
Thoughts?
Possibly create magical items that allow casters access to higher level spells for atunement and sufficient class level.
Spells are not really 9 levels deep. 1st and 2nd level spells share a similar power level, but there is a jump in power, generally speakimng, with 3rd level spells. Then, 3rd through 5th level spells are similar in power levels with only slight gradiation, but there is a substantial leap again with 6th level spells. 6th through 8th are one more grouping, and then they give 9th level spells a bit of a bump again. I've tested out eliminating the 9 levels of gradiation and only having 4 spell levels where 1 and 2 combine, 3 through 5 combine, 6 through 8 combine, and 9th remains alone - with the new lowest level spells available at 1st, the next tier available at 5th, the third tier at 11th and the final tier at 17th ... and then just compacting all the slots available into these 4 tiers. It tweaked the balance a bit (there was less 'upcasting', people could spam 2nd and 5th level spells), but it was not actually that significant of a change.
To that end, such a system as you propose might as well go up to 5th level spells. What 5th level spells break the bank for you? And do you really want to eliminate Raise Dead?
To me, the solution to these perceived balance problems are mechanics added to martial classes that allow them to physically engage with their environment better. The prootype for a high level fighter, barbarain, rogue, or other physical fighter would be comic book characters like Captain America, Green Arrow, Hawkeye, Black Widow, Black Knight, Batman, Shang-chi, The Punisher, Iron Man, etc... When they fight, they use their tools well, but it is often the way they use their environments to their advantage that is evocative and powerful in combat. They throw sand to blind, flip tables onto foes, swing from the chandeliers (from the chandeliers), etc...
If I were building the 6E martial classes in an effort to 'bring them up' to spellcasters, I would:
1.) Give them environment interacting abilities that allow them to apply conditions to enemies, alter the battlefield, and provide other battlefield control elements. However, it would not be how it was done in 4E.
2.) Give them greater durability than spellcasting classes in terms of saving throws and resistances. Most of these would be treiggered abilities they could call upon a limited number of times to give them the feel of them digging in and calling upon their inner reserves as opposed to passive benefits that we take for granted.
3.) Give them legendary actions as they advance in level, allowing them to do more in combat over a round. Fighters would get to do more damaing attacks (attacking faster would be their schtick), barbaians would be able to break the environment around them and provide battlefield control, rogues would be able to control their place in the battlefield (by moving, avoiding, hiding, misleading, etc...)
4.) Putting the damage potential of melee classes per round on par with the better spells of the spellcasting classes. A fighter, rogue and barbarian should be able to doeal as much damage as a disintegrate spell when it becomes available to the wizard.