In terms of game balance, it is hard to say how a character with significant opportunities to make things to 'boom' accidentally should be built, as it has a good deal with both how the player role-plays the character, and how the GM/Storyteller handles the character as well.
I built one such character for the playtest, and subsequently annihilated one square block of San Fransisco by accident...
In my case, I had a character with high Conviction (Superb +5), Evocation, and essentially no Lore or Discipline. My character also had a number of Aspects which revolved around his latent talent/power and complete lack of training.
With the idea of using Thaumaturgy/Ritual Magic and Enchanted Items or Potions to allow a practioner to 'cast' Evocation-type spells, I can see how that can be done, but with some fairly strict limitations.
For Enchanted Items, the effect (Fire blast, Spirit force shield, etc) and power of the item must be chosen at the time of item selection. Additionally, the Power of the item is limited both by the size of the item (and therefore the number of Enchanted Item slots it has...) and the Lore of character, with a maximum Power of Lore x2. In order to get the item to that level, the character needs to devote Enchanted Item slots specifically to increase the Power of the item beyond the character's Lore skill. If a character wants to subject themselves to Mental stress to get additional uses of the Enchanted Item beyond whatever number they would normally get per session, that if fine by me, but they are still limited to whatever pre-selected magical effects the player chose to equip the character with.
For Potions (which are really just consumable enchanted items) they can be chosen at the start of a session, or on the fly via Lore rolls and/or Fate point usage. However, these can't be used repeatedly by having the character take Mental stress, so no more Potions than the character has Potions slots and the effect is still limited to a max strength of Lore, or Lore x2 if Aspects were invoked during 'Potion' creation of use, as well as being limited to whatever effect the player chose for the potion to be.
Additionally and it is very important to note, if the character does not have their Enchanted Items or Potions with them, they are completely incapable of achieving Evocation-type effects since they can only perform Thaumaturgy/Ritual Magic.
With that last restriction in mind, if the idea is for a character to be able to handle significant Evocation-type castings when properly equipped, but still able to cast (albeit have no control over their magic when not equipped,) it might make more sense for the character to have either Channeling, Evocation, or a relevant Sponsored Magic instead of Thaumaturgy/Ritual Magic.
-Cheers
