I was amazed at how similar the animation style was to Filmations Flash Gordon, which I faithfully watched.
Well, they do share a common house style to an extent, but the animation on Flash
was a lot more sophisticated and elaborate -- since they'd originally made it as a TV movie (which, tragically, has never been rebroadcast or released on video since its single 1982 airing) and recycled a lot of its more elaborate animation into the episodes of the show.
What I don't understand is why nobody seems to notice that TMP stole just as much from TAS One of Our Planets is Missing as it did TOS Changeling. A large space cloud threatens to destroy a Federation planet. The Enterprise flies to the center of the cloud where Spock mind melds with the intelligence behind the cloud.
Finally! I've been making that point for many, many years. You're the only other person who ever seems to have noticed.
Indeed, I used to assume that since Alan Dean Foster wrote the story for TMP, he was directly influenced by having novelized that episode. But when I read the film's story outline and script drafts in the book Star Trek Phase II: The Lost Series
, it turned out that most of the similarities to "One of Our Planets" were absent from Foster's outline and only showed up in later drafts of the script, which Foster wasn't involved with. So I guess they're coincidental parallels.