That's why, back when I worked on a system that taxed health, the idea was to use a casting power gauge not too unlike a golf swing power gauge in golf games.

You press down until the meter reached the power level that you wanted.
Your skill in a particular spell determined how easy the meter was to use; the better the skill, the less erratic the meter line (a line that moved up the meter when you pressed to cast) moved.

If you wanted a quick spell, they were less powerful and only required you to click.
If, however, you wanted a really put a punch in the spell's power, then you would press and hold until you reached the power level that you wanted.

If you needed more power, but did not have enough to make the spell powerful, it would bleed over into health and tax it (the meter started off on mana [in the system stamina was not an option because it had other uses which impacted in ways that made it not possible for taxing]).

There were also spells which came with preset power levels, but they were not as dynamic as your spells, as your spells were made by you for your needs with five components of your choice each (spells were divided into sub-spells, or smaller versions, and broken into families of spell types and functions).

This meant that your needed spells for doing what you specifically like to do were the spells that required the use of the meter, while the spells made by us, the developers, were just quick and easy spells to use if you did not exactly want to use a meter guided spell (this also helped for magic users that didn't want to use the meter at all).

I say all of this to show how all of those conditions Edge expressed concerned over can be addressed without knowing the full concept details of what magic will be like precisely by allowing a variable use system.