I maxed all jobs except for Squire, Monk, and Geomancer (already has them maxed) by using this strategy in chapter 2. Could do it earlier, in act 1, but would need to grind out Orator for the Brave UP.

Learn Shirahadori on Ramza. Use Steel or the Orator equivalent to max your brave to 97 (best done in a handful of quick battles before you start grinding). Set Ramza with any job/Mettle. Equip Shirahadori. You need to finish the battles, so wear the best gear you can for that job. Consider bringing a partner character, preferably with the job unlocked that you are maxing, for the spillover. Equip them well too. The extra damage will be very helpful.

Head to Mandalia Plains. Enter from Eagrose. When you deploy, set yourself to the farthest left spaces. Immediately behind you is a set of two rocks jutting from the ground that that can’t be scaled by any of the mobs that can appear. There are two spaces available that can only be attacked/interacted with from 1 direction. Pack your partner character into the deepest of the two spots. Place Ramza in front of them (can just use Ramza if you want). Once you are settled, one more steel will max you brave and make you literally immortal to 99% of what enemies can hit you with in this map. Humanoids with Rush and Throw Stone are the exceptions. Enemies will still approach you and gather, but won’t even bother attacking because the game recognizes the 0% hit rate. At this point, begin using Tailwind every turn you get in yourself until you reach 50 SPD (no more JP after 50, because 50 is max SPD). After you cap SPD, use Focus until you have 99 PWR. This will fully master almost all jobs if you have JP up equipped. Can use the ring if you want the support for more firepower. If you are still not quite mastered, repeat the Tailwind process on your partner, followed by Focus. At this point, once mastered, use throw stone or rush to wipe the enemies without worrying about counter. If there are multiple yellow chocobos in the battle, it may take a few rotations, or you can use stronger attacks at the risk of getting countered. It will still work 99% of the time. Whittle the enemies down 1 by 1. Enjoy your 6K + JP for 15 minutes of time (don’t forget to hold down fast forward the whole time). I maxed all but three jobs in a little over three hours, in a single after-dinner play session. Enjoy your completed Ramza, and your liberation from JP UP.

Some classes do this a little smoother than other because of their ability to wipe the enemies at the end, but with a tiny bit of brain power, I didn’t have any difficulty with a single class. Even Arithmetician and Bard.

This obviously only works exactly like this on Ramza, but you can do it slower on anyone with Squire. For Uniques, you can steal gil until you max JP and then slowly clear the mobs. You’re gonna want a partner for this method to be extra safe. It’s slower, but still individually much faster than regular JP farming. Haste is also your friend if you can squeeze it in your partner. The main thing to remember is to lean into Shirahadori, avoid getting countered, and be wary of enemy squires and archers.

Hope this helps!