Derivation is a cornerstone of creativity
"... but also the content generated by those models – incorporating themes, elements, and styles created by human artists – was unlawful derivative work."
So, we (human, or a supposed AI) are meant to admire an artist's work, but woe betide anyone seeking to learn from it, to improve upon it, to take forward nascent possibilities, to express excitement via emulation of something momentarily original which itself inevitably arises from a long line of 'derivation'? Also, note that should something stunningly original arise, that in any area to which anachronistic copyright applies, then restriction on derivation threatens to leave a shining jewel forever (at least decades) detached from the evolving body of human achievement.
No combination of legal phrases can result in a clear distinction between 'legitimate' derivation and that which infringes so-called copyright. There is one protection against losing the opportunity to raise income by sale of original copies of one's works, and only one, for which an unambiguous law can be framed to benefit an artist; that is against misrepresenting oneself as the artist (composer, writer, etc.) to gain pecuniary advantage. For example, signing a drawing falsely under the name of the true artist, and seeking to sell it. Works clearly resembling, or derived from, those of another person, could then avoid accusations of plagiarism if offered to the world under headings such as "In tribute to X", "Based upon a work by X", "A variation on X's depiction of ...", "A digital copy of ...", and so on.