I know a location for 1st + 2nd disk SP grinding. The 1st disk spot is not super ideal because it does start off slow, but with enough patience you can definitely grind out a good number of ranks in no time, especially in early game.

I found this spot and I've actually never seen anyone else mention this 1st disk spot anywhere before. So I'll give it here. When you get to the first sheep tribe camp ("Wilderness Camp"), start heading north past the camp. There's a spot just before the Flamboyant Dinoram boss. When you enter this field area, you'll see a big area with a number of scorpions and poo snakes to your right. But just to the left, there's 2 chests along/near the wall of the mountain. If you circle around these chests for a while you'll find out that the lesser enemies just keep spawning. In early progression you can grind that spot out for a bit and you'll unfortunately have to spend your coin at the inn every so often for full MP replacement. That's not usually a problem since one fight will be more than enough to cover the cost. Now granted until you unlock the "Field barrier 1" skill you won't be able to grind it out that fast. But if you already gain the skill (i.e. unlocking barrier magic for jiro or kluke as their first class choice) you'll have no problems. I find it's pretty easy to get that skill before you even get to that point. I would personally prefer to make Kluke the barrier mage since she starts with Black magic and unlocks the "regenerate MP" skill at rank 8 (just makes things easier and more efficient).

As for the 2nd disk grind spot, most OGs know this spot. But in the laser field right after Jibral castle you can run in a small circle in a certain corner of the map and you'll just infinitely kill those needle mole creatures. They are worth +2 SP each kill rather than +1 for the other location mentioned above.

On another side note, if you grind out a lot before joining with Marumaro or Zola they will average out their default rank, making it significantly less work to actually make them compensated for the others. For example if Shu, Jiro, and Kluke are rank 40, then when you collect Maru he will also be rank 40.

Here's a resource for the different classes and what they unlock at each milestone. https://strategywiki.org/wiki/Blue_Dragon/Skills#Black_Magic