The martial/caster disparity is the (IMO, proven and obvious) idea that martial characters lack gameplay options compared to their caster counterparts, and that this problem only ever gets worse with level.

Also, "martial" in this case specifically refers to Fighters, Barbarians, and Rogues.

IMO, it does exist, but it's not as "end of the world"-bad as some people make it out to be. Basically, rogues are fine because they get a crap-ton of skills that can be put to good use as long as the rogue player makes their character with even a little long-term thought. Rogues that have problems tend to focus in things like stealth, and other physical skills that casters can use spells to imitate or replace. Rogues that pick up and spend expertise in one or two soft skills (some kind of knowledge skill, insight, investigation, etc...) will never find themselves with nothing to do and will always have a niche where they can make the full casters go "holy shit!" from time to time.

Fighters and Barbarians actually have problems because they seem to have been made more with dungeoncrawling in mind, to the detrement of anything non-dungeon related. They generally lack useful soft skills, and don't stack stats that will make using them useful because they generally don't have ways to make a high int or wisdom terribly useful.

Fighters and Barbarians compound the skill problem by not gaining useful/impactful abilities in T3 or T4. When full casters are busy choosing and enjoying the most powerful spells in the game, fighters get another use of indomitable (which never, ever fucking works, IME), a second action surge per short rest WAY too late for it to really matter, and a 4th attack they will probably never, ever actually get because it's at 20th level for some stupid, fucking reason (as opposed to level 17 where ALL casters, even the half-casters, get their 4th cantrip damage die).

Barbarians get even less than fighters due to most of their class budget being tied up in a massive passive ability: Brutal Critical. So all they ever get to do is crit-fish, which they've all been doing since level 1 anyway.

The disparity is choice and impact. Because of their lack of choices, it can seem difficult to have an impact on the game, mechanically. A good DM can make up for this in a variety of ways, but when you're just looking at the rules or white-rooming a character, the problem does tend to become a bit obvious...if overblown.

Generally, the fix is simply to give fighters and barbarians more class abilities that involve getting to make interesting choices during play.

No I absolutely agree with you on both of those points - Martials need more resourceless abilities, and casters are too strong and nerfing them is an absolutely essential step. People are too resistant to nerfs, but PF2e casters are just as competent as ever - they were made less omnipresent because the problem isn't just in how absolutely shafted martials are(though that is definitely a big, BIG issue that WotC seems to not even be trying to hide anymore), it's the fact that magic is too cheap for such powerful effects, while martial abilities are just as if not more expensive for less.

Honestly, the main example I turn to regarding both of those points, and WotC absolutely hating martials, is one single ability - Commander's Strike.

When you take the Attack action on your turn, you can forgo one of your attacks[1, ah ah ah] and use a bonus action[2, ah ah ah] to direct one of your companions to strike. When you do so, choose a friendly creature who can see or hear you and expend one superiority die[3! ah ah ah]. That creature can immediately[This Count Dracula joke doesn't apply here...] use its reaction[4! 4! Ah Ah Aaaahhh!] to make one weapon attack, adding the superiority die to the attack's damage roll.

Have you figured out what the muppet vampire was counting? That's right, Resources spent by this fighter ability! And the total count is FOUR. FOUR ENTIRE RESOURCES SPENT BY THE PARTY, THREE OF THE FIGHTER'S AND ONE OF THEIR ALLY'S, ON ONE ABILITY. SPELLS NEVER COST MORE THAN ONE SPELL SLOT EVEN AT 9TH! WHAT THE FUCK?? Let alone the fact that all you're getting for it is...A single extra attack from an ally that turn. With a bonus to damage, sure, but that only applies if it hits. And to add insult to injury, it says "immediately," so the one getting it can't even delay it to decide the optimal target or save it for a particularly sticky situation.

The game and its players seem to just put more expectations on the non-magical people for them to be able to do anything than casters, who can just do things with no strings attached.

Most spells also used to cost something to cast other than a spell slot. When your only limitation is 8 hours of rest, magic scales wildly out of control.

This post I made just...keeps coming back, doesn't it?

Or play a system with better design instead of trying to force the wargame to be anything other than what it was designed to be.

Okay fair, I suppose. That is part of why I mention Pathfinder in the post, though, so...Eh. Whatever.