Alternative theory: Chiba has been using his previous life's middle school chunibyo phase magic system as a framework for his spells, but they're very a inefficient way to use mana and the "200 year lifespan" idea cut his mana capacity in half and did nothing to his lifespan. It's like Myne from Ascendance of Bookworm, throw enough raw mana at a problem with a vague understanding of what magic is and hope for the best.