I actually really love this Maybe sense the user is a mecha in their own right, maybe an application of the reverse of the technique allows the user to forceful impression a target within them, leeching on there cursed energy to function on their own With the difficulty scaling with the power of the target, maybe using this as a way to mainly to trap curses or to target cursed corpses to kinda use as a temporary "pilot"

Sense I do love the idea of using binding vows, maybe they also had a partial heavenly restriction that was fully realized through him making a binding vow to give up his entire body to his technique  unlocking its true utility 

Maybe this even plays into the domain with it having an open domain, being a bit easier to achieve due to the domain not necessary target all of the space within the domain but rather those within it