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I don't use Plex any longer for this reason. It's a shame because I bought a lifetime Plex Pass a few years back and was great until devices decided to update and become unusable..
If you have to use Plex I would probably go the HTPC (Kodi+Plex) way. If the latest Kodi is uncomfortable you could install an older version or set it up to use an external player e.g. a good build of VLC/MPC-HC. It doesn't look as pretty as the Plex apps but it just about does the job.
FWIW this dilemma isn't just facing Plex users, but any app-centric service like Netflix, Prime Video will all soon need to be on the latest tech to work.