Even though the movies are mostly self-contained long episodes, there are some signs of continuity between them and the anime. Usually in the form of which Pokémon each character has caught and evolved so far. In practice this does not affect the watching experience much, but you can follow this order if you care about that.
But rarely, the anime and a movie can make references to each other. In the case of the first movie, this placement must be because
spoiler
During episode 65, Mewtwo is seen escaping from Team Rocket HQ, and the movie is implied to take place some time after that as Mewtwo has been free for a while at that point.
I also noticed this watching order is missing the newer movies (“I Choose You”, “The Power of Us” and “Secrets of the Jungle”). If you intend to watch the complete Pokémon anime I don’t recommend skipping them, they’re pretty neat. Each of those is its own separate timeline and they have no shared continuity with the anime or each other, so you can watch them whenever. But my advice is to watch them alongside SM and Journeys just so that the difference in production value between the movies and the anime is not too jarring.
Also, I assume you already know this, but the Pokémon anime is still going. Horizons is there, it has no relation whatsoever to Ash but it comes after Ultimate Journeys, and it’s mostly fun to watch.







I thought this was a bug of Summit, the app I use.
Sometimes I see an interesting top-level comment, I tap on it to expand the replies while still reading, and suddenly the top-level comment is gone. Removed. I assumed it was a bug because it does not make any sense to me and I keep seeing this happening everywhere.