Whenever I play any Ultima game I try to give my Avatar a solo-mage bent, but in U7 it is pretty difficult.
Obviously the mana limit (as you found) is pretty tight, but even worse is the cost of reagents vs the actual usefulness of damage spells. Because it's harder to get money in U7 compared to the previous games (without cheesing gambling, that is) if you're relying on Fireballs in the early game you're going to be out of reagents quick and without a good source of money to buy more. So for the early game relying on martial combat is a necessity.
Plus even when you get spells more useful than Fireball, like Death Bolt, they're still often let down by the Avatar's abysmal aim. Nothing is more frustrating than watching your mana and reagents being quite literally thrown into the wind as the Avatar fails to Death Bolt an enemy less than 3 yards away.
So until you get Death Vortex I'd say your best bets are actually Energy Mist -- which deals a huge amount of damage with no need to aim -- and Magic Storm. Magic Storm is especially useful for killing things like dragons through walls! The big problem, of course, is even surviving until you can get those.
Another spell that's sometimes useful is Charm. The amount of time the target is on your side can vary wildly, but a good charm spell can rapidly turn the tide in your favor -- in particular I have a fond memory of an entire squad of guards being taken out by one of their own. Paralyze can also be useful if you're willing to beat your target to death or pelt them with wand blasts.
Theoretically I guess you could also make an archer/mage hybrid, since enchant only effects arrows in this game. Not really sure how effective that would really be though, and obviously it's not a "pure mage" anyway. Though, really, even a "pure mage" is probably going to have to bash some heads the old fashioned way.
That's just my ramble-y 2cents. U7 is my favorite in the series but it is unfortunate that my preferred "class" is really gimped compared to U6 and Underworld. Alas, no game is perfect.
Post edited March 19, 2019 by BelatedGamer