I understand that it’s probably complicated for Plex to provide a universal solution for all the devices out there, but I was actually surprised by how much easy it is to fix from the user side: it looks like just editing one line in the server profile of your device makes sync not transcode anything you don’t want it to.
I hope you’ll find the following links helpful: original solution for docker container, and same with windows Plex server, and on Synology you can find profiles under /volume1/@appstore/Plex Media Server/Resources/Profiles/ (you’ll have to use ssh to access that)
I advice to list only the codecs that can be direct played from your device, otherwise you might find playback issues.
I only tried it for iOS, but it’s quite possible it’ll work the same way with Android profile.
I only tried it with one HEVC file, cause I have no need for sync currently, so there’s no guarantee it’ll work at all or won’t make things worse, so remember to back up the original profile to roll back to it.
Also if you have multiple Plex sync devices and some of them aren’t compatible with added codecs, it might affect them if they use the same profile file. I’m not sure whether it’s possible to create new or fine-tune existing profiles for such situation.
Also, future server updates may overwrite it, but still much better solution than nothing
UPD: useful info on writing profiles in general: Writing profiles for DLNA devices
