A looooooooong time ago I struggled with what you are experiencing now. I came to the conclusion that the only way to fully control my artwork was to include it alongside my movies and tv series.
Its certainly a bit of work and can seem daunting for mid/large libraries but once done it works perfectly and you’ll never look back. I just took it as a couple of movies each day and before I knew it I was done. Both https://theposterdb.com and https://mediux.pro have amazing artwork contributors.
BTW… if you ever manually selected a poster in the plex UI then they maybe considered locked. To fix this use this procedure Reset all manual poster selections in a library at once or unlock other metadata fields. I however, fully rebuilt my library once I completed this work as I did not want to risk any bad data in my db.
Anyhow, food for thought!
