I deeply hope 2024 combs thru all of the spells for very tight balance.
Generally, currently:
Slots 1 and 2 include a variety of powerful spells. So in practice, there is a smooth scaling of power from slots 1 to 3. Fireball is better to avoid "special" treatment, for the sake of all spells in the same slot having a clearer amount of expected power compared to each other.
Slot 4 is slightly less impressive, but slot 5 gets a boost in power to recontinue the smooth scaling since slot 1.
Generally, slots 1 thru 5, are well understood. The spells still need calibrating to ensure each spell is performing balancedly and desirbably for its slot. But overall, these slots are solid.
However, slots 6, 7, and 8, are increasingly disappointing and actually yuck. Except, a few spells are ok but require foreknowledge, planning, and setup in advance, thus are less usable for the spontaneous spell slots. Exceptionally, Forcecage is actually broken. But slots 7 and 8 are truly horrible.
Then slot 9 includes spells that are runaway throwaway broken. Albeit, there are also dreck spells that pollute the slot 9 with worthless spells like Storm of Vengeance.
Wish is too powerful for a slot 9 spell, and is an example of what not to do. Probably Wish should be an "Epic tier spell", and only obtainable before then by using the level 19 Feat to obtain it, thus casting it "Innately" once per day. Every full caster can qualify for the Wish feat, whether by psionically willing the minds intention into existence, or divinely accomplishing a miracle, or arcanely reconstructing reality, or primally animating nature. Meanwhile, the other level 19 feats need to be worth the same amount of power and desirability as the Wish spell.
The slots 6, 7, 8, and 9 are in desperate need of designer comprehension and radical regulation. The game engine needs to completely understand how much power these high tier slots must supply. Every spell in a same high tier slot must be equally desirable, while scaling smoothly from the slot below it to the slot above it while advancing.
When slots 6 thru 9 are finally working properly, there will be a clear understanding of how "mythic" the Fighter class needs to be at the higher tiers.
Then there will be enduring game engine balance, even at the highest tiers.