Agree that Mana Points might not be the way to go.
I would have Magic as its own skill and let the different spells have different difficulties. So you need a number of successes to be able to cast a certain spell.
Maybe different schools of magic would require different talents to unlock the spells of that magic school?
A player who chooses the class Wizard then gets one magic school from the start. Could also unlock a second magic school from the start by using the third (free choice) talent.
This system would also allow a Fighter to choose a magic school as the third (free choice) talent and set some points in the "Magic" skill allowing for a little bit of "multi-classing".
Whilst I think this is in concept a good solution, I don't like that rolling successes on the dice might mean you still don't succeed. It's one of the biggest criticisms of a similar system I've played (d10 dice pools, but difficulty determines the number of successes required). Basically, if the dice turn up a success, it should mean success.
One way around this might be that rolling less than the requisite number of successes allow some smaller effect to occur (a partial success, if you will), but this feels a bit too crunchy for me.
I'd rather the dice pool gets modified with fewer dice (my earlier suggestion), which fits in with the existing MYZ difficulty modifier mechanics and still honours a single success in the dice pool.
I make YZE games (https://drivethrurpg.com/browse.php?author=Matt%20Kay) and produce predominately Free League content on my YouTube channel (https://youtube.com/@3skulls)