My Daggerfall mage (level 4 argonian) at this point can take care of most creatures with fireball+spell absorption. Of course this has the effect of leveling up my destruction skill faster than any other, despite the fact that I use levitation, water walking and water breathing and slowfalling a lot too (and somehow those skills barely ever level up). Are there any good spells I can custom-make for mysticism and illusion that I can use regularly to help level those skills up too? I haven't messed with the spellmaker much yet because I don't want to make a spell too strong to cast/burn up all my magicka or too weak to help at all.

What you can do is create a "training spell" for each school. Create the cheapest possible spell that you can either cast on yourself or as a ranged missile (so that you don't need to touch an enemy to use it), hop in an inn or camp outside and use the spell until your spell points run out. Rest, rinse, and repeat.
There is a chart on UESP.net that lists each spell according to their school.

If you are fighting your way through dungeons using destruction spells you could try using the spellmaker to spice those destructive spells with extra effects. You can add up to 3 effects per spell. For example light 1 second is cheap and trains illusion. Other choices that come to my mind at the moment heal stamina 1point (restoration), levitate or free action 1sec (alteration), silence or soul trap 1sec (mysticism), water walking 1sec (thaumaturgy). If you make the spell raise +1 per 2 levels or more the spell becomes even cheaper to cast and produces the same training effect.

For practical spells, if you want to cast water walking it's better to make a separate spell that combines water walking and water breathing, but don't make it too expensive.
These spells are additive so if you cast them more than once with the Q key for example you can keep adding to the duration and the training effect is greater because you will cast those spells more times.

Cool, I didn't know they were additive. Training spells it is, then. I want my skills to be somewhat equal so i can have the option of sneaking by with invisibility or absorbing an enemy's health rather than machine-gunning them with fireball all the time.