Grandia series. I'm partial to the leveling systems but the battle system is fantastic. There's ATB, field movement, charged attacks/magic incantations that can be canceled, a light combo system. I just love it

Yes it's proportional, someone with 16 SPD will act twice as often as someone with 8 SPD. Of course turn order is affected by other things like charge skills and status effects, but having crazy speed (it caps at 99) will allow you to have like 8 turns in a row. It's broken and available from the start.

I'm lazy and I like to have Ramza buff himself by switching his AI to protect himself. Expect Ramza to level up a lot with this strat

You can earn JP in random battles. Yell increases a character's speed by 1 during the battle, but it stacks. After using it a few times, Ramza will play multiple turns in a row.

He has another skill that does the same, but for strength. It's very cheesy but it will make you win fights.

Ramza has an ability called Yell or Tailwind (depends on the version you're playing) in his default class. Spam it on himself and enjoy the cheese

I also really liked them, novellas included, except book 6. That one just didn't work for me (and apparently for quite a few people), and I ended up reading a summary.

Since books 7 to 9 aren't included in the show, they're worth reading just for the continuation and conclusion of the series.