Unstable Mana: magic system variant.

[Giovanni]

Does producing this pool cost FP (requiring you to rest for 20 minutes per die generated, meaning magic users will wake up, then pretty much immediately have to sit and rest for a while), or is the amount of FP the character has simply serving as a limit?

It should cost FP. It should serve as a limit. But maybe it's not a good idea. Maybe, in the morning, each magic user sould choose between 4,5,6,7, or 8 dice mana pool.

Next, when casting, if you don't generate enough energy for the spell on this roll

The spell fails and, in order to retry, you need another action.

Is the energy generated by your mana pool free, or do you still need to pay FP for each point of energy?

It's free.

Do the random effects occur immediately

Immediately.

My immediate reaction is that I wouldn't want to use this system at all - not only because I tend to have poor dice luck (and thus would likely lose my pool rather quickly) but also because these appear to do the equivalent of a critical failure at least 17% of the time and typically a lot more (the average of your suggested range, 5, has around a 60% chance of rolling at least one 6

It's not intended to use the critical failure table. Random effect should be hilarious (think about wild sorcery in D&D 5) and maybe a bit annoying but not life threatening (unless you roll a lot of 6 of course).