I'm interested in devising a homebrew system for spellcasting beyond one's regular capabilities. Will this description be unambiguous and keep relative power balance between non-casters, third-casters, half-casters, and full-casters?
Likewise, are there any ways to mechanically exploit this system, and are there any revisions that allow for greater clarity of the system itself?
If in dire need, any spellcaster is capable of expending their own life force to enhance their connection to the Weave. This allows them to cast one prepared spell without consuming a spell slot.
When you do so, you must make a DC 10 + Spell Level Constitution Saving Throw. On a failure, you take a number of levels of exhaustion equal to the spell's level immediately after the spell is cast. On a success, you take a number of levels of exhaustion equal to half of the spell's level, rounded up, immediately after the spell is cast. If the saving throw is failed by 5 or more, you take the levels of exhaustion immediately and the spell fails to cast.
You can only Overcast spells at a Spell Level less than or equal to your highest level spell slot plus one, the total of which is no higher than nine.