Tom I know you’ve been trying this with Plex, I’m trying to do the same with Jellyfin, an Emby fork.
I have Beelink J45 Apollo Lake CPU device, installed minimal Debian Buster and then the DietPi script to convert.
I installed intel-media-va-driver. Set the environmental variable LIBVA_DRIVER_NAME=iHD.
I used this guide from Jellyfin to enable transcoding.
No luck if I playback any HEVC encoded video with transcoding enabled.
Any advice on how you got it working?