Eternal Spells Should be Improvable or upgradeable Somehow

I only really use eternal spells because it is annoying to farm spell materials and I just don't like the idea of using consumable spells. I would like Eternal spells to be on par or better that their consumable counterparts. You should be able to level them up somehow.

To do that I have a few ideas, firstly they should all have their level capped by the player to prevent over leveling and that seems like the best way to do so.
My personal favorite idea for how they would level is to have them level up through use and xp because it is more combat/exploration based than material based.

The second idea I have is to just use runes/legendary runes. This is more material based but the material is something you usually get from combat and exploration so it isn't too bad, but it seems less interesting to do and a lot of people have huge rune stockpiles. It does seem easy to implement and in line with other weapons.

The last idea is to just use regular materials. Like maybe the shroud drops or materials that would make the consumable spell variants. I personally don't like this one as it seems quite grindy.