Why does the martial/magic divide need to be "fixed" in the first place?
Casters have a much higher damage output than Martials, making Martials less useful and feel like they have less agency
Because generally accepted adventuring practices (splashy 2-3 combat difficult combat encounters packed in a 4 hour session then long rest) hugely deviate from design intent (6-8 resource draining encounters per Long Rest)
In this style of play, Wizards and Magic-Users are problematic because they are consistently powerful and efficient (difficult encounters decrease chance of "overkill" and spell waste). This issue is compounded at open tables (AL) where characters need a reset between every session for logistical reasons.
So there are few ways to remedy this:
1. Tinker around with spell slot numbers
Technically, this should be a sound solution, but I do not like this. While this does reduce the total damage output of Casters, this discourages utility spell usage even further, reducing spell diversity and caster agency. While restricting agency does fix the problem, I think one should aim for a fix that does not do this.
2. Run more encounters per long rest
Easiest way, but not always feasible due to meta reasons
3. Scale up encounters appropriately if you have less of them
Instead of messing with the numbers on the PC side, maybe DMs would be advised to acknowledge the damage potential of Casters by making the Encounters you do have, more challenging. Adding a CR (fuzzy in math but something in that area) per caster in the party if you don't have many encounters. In this solution the Casters will still be powerful, but be more dependent on Martials to succeed together because their resources are taxed more.