> I made minor modifications post-generation (open question, probably enough)

Hmm I don't think that would be enough. I'd expect that would only make the modifications themselves copyrightable, but not the whole modified work including the AI parts.

Compare for example the case where the US copyright office ruled that, when assembling AI-generated images and human-written text into a comic book, only the human-made elements themselves (text, arrangement) get copyright protection, but not the images.

E.g. if you generate an AI photo and color grade it, i expect only the color grading would be protected (if that is even significant enough to be protectable), not the rest. And someone else could re-color-grade the same image without infringing your copyright.