Niggross
i still hope you go with rebellion due to its concept of unification which for his situation is far better in the long run than yamato that weilds the concept of seperation. with rebellion it could fuse his mana sources into one so instead of only a 1/3 of his overall mana being used for a single magic system he could use all of it. and he could fuse the magic systems together allowing new types of spells that none of the 3 could do on their own. the physical side would also fuse so instead of him just being an amalgamation of races he could be a perfect fusion removing the confliction of the racial changes making him not aquire the real power of the physical race. Finally rebellion's fusion would also give him a whole brand new sword that is specifically attuned to him and can be summoned/unsummoned at will unlike yamato that can be stolen. The new sword would be unique to him and wouldnt just be a copy and paste weapon which would make him stronger and give the weapon unique abilities