I've discussed implementing a mana system with my players and will try to use this in our next session. At first glance it sure seems strong, especially around the mid levels - they're lvl 10 right now - but I love the idea of giving them more versatility. How are your experiences with this? Have you thought about adding something like mana potions to give them more sustain? I am aware that this will make balancing dungeons way harder but at the same time, if one uses a mana point system, I feel like there should be options for the players to refill their mana.

I've been tinkering with a feat using your mana system and would be curious what you think. The idea behind is is pretty much to give even more relevance to mana and also provide a system to restore it that in my opinion makes sense.

Whenever you cast any spell of at least level 1, you can try to imbue it with additional powers. Roll a d20.
1 - 5: The spell fails and costs the normal amount of mana points.
6 - 10: The spell works as it would normally.
11 - 15: You can add your proficiency bonus to the spell's damage. (OR: allows you to add your current caster level + proficiency bonus as extra damage for the cost of the same amount of mana points).
16 - 20: The spell is cast at the next higher level - if you choose so - for the original mana cost of the level you cast it on.

This feat allows the caster to treat his mana pool in the same way his HP pool is treated. It adds the HitDie the character is using as ManaDie and works in the same way. Meaning, the caster can use his ManaDie on a short rest to restore some of his mana.