My advice and again take this as you will 
If you care about artwork then just don’t let plex manage it. They announced a while ago Movie artwork updates the outsourcing of movie posters to gracenote so plex themselves have walked away from artwork and are letting them manage the defaults.
I would suggest using https://support.plex.tv/articles/200220677-local-media-assets-movies. Both https://mediux.pro and https://theposterdb.com are excellent resources for quality artwork. Will it take you work and time, yes, most certainly. However, just take one movie at a time, even a couple each day and before you know it your done with your own high quality artwork. Your future self will thank you
