Just a note on the cantrips — the 5e spellcasting and combat systems pretty much expect that magic-users can cast unlimited cantrips, giving a magic-user a way to contribute to a fight even once they can't cast any more levelled spells. We wanted to keep a bit of that (while still making it clear that magic, especially unstructured or unfamiliar magic, causes Corruption). So, as a magic-user becomes more experienced, they are able to manage the Corruption from cantrips better and better, eventually reaching a neutral point (whatever Corruption gained is effectively immediately dissipated). This allows us to keep a lot of 5e expectations that factor into the combat system and challenge ratings while still fitting the theme of Symbaroum.
I guessed it was kept like this for balance issues. It's more a personal pet peeve of mine with D&D 5e that you will never reach the desperation as a caster that you are "out" of magic, and need to use that dagger/quarterstaff/whatever. I do really like how your implementation of the corruption system looks so far with the risk vs reward of pushing your luck with spells when you have already cast a few spells.