Reloaded a decade-old save, where someone told me he was surprised I beat the game as supposedly I was too underleveled, and testing around with the small materia build I came up with, I think remembered why:
Thundaga iirc I found lying early in the final dungeon, and Osmoga about halfway through (correct me if I'm wrong, going by memory here).
Thundaga is the fastest of the -ga spells I had found, it goes through enemies for a decent chance of chaining attacks, and does a lot of damage. And Osmoga is there because Thundaga drains Zack's MP way too fast.
Also I had equipped Cura and Healing Wave since enemies were hitting like a truck, some HP and magic materias for surviving and damaging, and (I don't remember why) Lucky Stars.
Testing with a few missions, which I totally didn't learn just now how to access, Zack doesn't even flinch before the enemies go down. Also yet to find any enemies immune to thunder.
I have some vague notion of how powerful a certain superboss is in the game, so likely it's not enough for that, but if you're struggling with the main game or some intermediary missions, maybe worth trying that set of materias?
Also no idea how it goes with the Steam remaster.
Hope it can help someone. =)