On Plex Android clients, SRT, VOBSUB, & PGS subtitles direct play.
ASS subtitles are currently unsupported and force a video transcode. However, Plex is adding ASS subtitle support to the Android clients. Unknown when it will be released (discussed in this thread).
One other thing to remember is that the Shield Pro can run Plex Media Server. It also has hardware acceleration enabled by default, no Plex Pass needed.
You could run PMS on the Shield, using SMB mounts to access the media on the Synology NAS.
The Shield will not transcode & tonemap 4K HDR, but might work quite nicely for transcoding 1080p SDR for remote devices.
It is not a problem to have multiple Plex servers on an account, so you could still run PMS on the Synology if desired.
