No it’s not cannibalism. That’s just not the right way to think about it at all.

And while the transsubstantiation as a specific word referring to the catholic sacrament is a triple miracle mystery, substantial change in and of itself is pretty much part and parcel of life as physical being right?

I do not think you meant differently but I did not understand the frankenstein reassembly reference so I wanted to clarify as it might give the impression substantial change itself is exotic.

After all even a non sacred piece of bread eaten as a remembrance started as its own substance, bread, and then substantially changed into you. It ceased to be bread and became the substance that is you soon after you ate it. And you are not some frankensteinian assembly of substances of all your past bread pieces eaten, you are just you.

The substance that is a human has that transformative power over bread, bread does not have that power over humans. Nor can bread resist being substantially changed into you. Bread cannot resist the transformation once you consume it.

“You are what you eat” is thus quite often used wrong; rather what you ate “are now” you.

So (mere) humans can effect substantial change all the time. That power is evident within us (albeit with limitations).