Which games, besides DOS and Pillars, have tons of skills and spells you can use?

Like the MM series? Preferably 1st person dungeon crawlers? Or strategy games?

Daggerfall has a bunch of spells and now has that excellent unity mod: https://www.dfworkshop.net/.

While it is not exactly what you ask for, there is Noita, which is on sale now and basically consists entirely of messing around with spell combinations: https://www.gog.com/game/noita

I also felt I got to mess around quite a bit with my spells in Arcanum, although it is not first person.

Bard's Tale 3 has a large number of spells. Also, some of the Might and Magic games (particularly Xeen) have a decent number of spells, and there's also Wizardry 6-8.

Moving to other RPG types, there's Fell Seal: Arbiter's Mark (which is obviously inspired by Final Fantasy Tactics); the DLC even adds more skills (especially if you take advantage of the ability to recruit monsters and get monster variants that are added by the DLC).

Grimoire and Kinghts of the Chalice

Carrying on from Daggerfall, Morrowind has 136 different spell effects, each of which can be combined in any combination you wish (depending on if you have the money to buy the spell and the mana and skill to cast it).

A little Alchemy can solve all those problems, if you're so inclined, allowing you to potentially cast spells up to the limits that the game will allow.

(Of course, if you do this, you have already broken the game, though you don't need to go far enough to trigger attribute overflow to get to this point.)

If your Morrowind character doesn't fly over the entire game world in seconds at some point, you are not playing the game correctly.

For spells I would recommend Tyranny or Two Worlds. In both games you can create your own spells, tons of them!

Not what you're looking for, but:

https://www.gog.com/game/magicmaker

If your Morrowind character has always had enough Magicka to use their greater power, you are not playing the game correctly.

(Greater powers cost 0 Magicka, but have a once per day limit. I'm talking about not trying to use it before 24 in-game hours have passed, but rather, I'm talking about not being able to afford the 0 Magicka cost.)

(Also, this "not enough Magicka for greater power" can also happen in Oblivion.)

Lichdom: Battlemage. You create your own spells and use them in a 3D first person real time environment.

Pathfinder: Kingmaker
Neverwinter Nights 2 Complete
Icewind Dale 2 Complete
Wizardry 8
Baldur's Gate II: Enhanced Edition
Baldur's Gate: Enhanced Edition
Neverwinter Nights: Enhanced Edition
The Temple of Elemental Evil
The Elder Scrolls III: Morrowind GOTY Edition
Dragon Age™: Origins - Ultimate Edition
Darklands

Pathfinder: Kingmaker and Neverwinter Nights 2 Complete are the kings, I believe (сonsidering all classes and the whole party). Icewind Dale 2 is the same.
DOS and Pillars are just a lite versions of this. Wi - zar - dry - 8 ! ! !

Morrowind is a 1st person and like a MM. It "have tons of skills and spells", but most of them are broken.
You've probably already played it.