Does Mitsuru's Theurgy Scale Off Of Only Strength?

No. Blade of Execution also sucks.
Going from 66 to 99 magic is about a 12.5% increase in damage for both Theurgies. Raising strength shows more value since Mitsuru's strength is not that high, so by maxing it you'll get more of a return. The damage does 100% scale off of both stats though.