You’re running into multiple issues: 1) Lack of PGS subtitle support in the Plex app; 2) Low power CPU in the NAS; 3) No HDR tonemapping in the Synology version of Plex Media Server.
Your choices are essentially: 1) Use only SRT subtitles; or 2) use a different Plex client that supports PGS subtitles (such as Android TV).
Subtitles:
In my experience, the subtitles from OpenSubtitles are of little use. As you’ve found out, there are many versions for each movie and most do not sync with the movie on your server.
An alternative is using tools such as Subtitle Edit to convert the PGS subtitles yourself. Since the source is the movie on your server, the resulting SRT subtitles will be in sync with the video. The downside is this is generally a time consuming process. Subtitle Edit may (will) pause frequently asking for assistance with words it cannot recognize, such as character names.
Note: “Forced” subtitles, such as when the characters in Avatar speak Na’vi, are usually easily converted with SubTitle Edit. The human intervention is still required, especially for items such as alien words. However, there are usually not many lines to convert, so the process goes rather quickly. For example, in Avatar, there are ~1800 lines of dialog. However, the Na’vi subtitle track contains ~100 lines, so conversion will take much less time than that for the entire audio track.
You can also manually download subtitles from other locations, load them into Subtitle Edit, correct the timing, then add them as external subtitles to your server. This can also be a time consuming process. See Fetching Internet Sourced & Using Your Own Subtitle Files, for a list of sources.
Plex Clients:
The Plex SmartTV app for Samsung (and LG) TVs do not support direct playing of PGS subtitles. When enabled, Plex Media Server must transcode the video and burn the subtitles into the video stream. The Celeron CPU in Synology NAS simply is not powerful enough to burn subtitles on a reliable basis.
Not all Plex clients have such restrictions. As you’ve found out, the Plex Android TV app can direct play PGS subtitles.
There are many Android TV devices available at different price points. The Nvidia Shield Pro is at the high end, ~$200 USD. The Amazon FireStick 4K sells for ~$50 USD. Both direct play PGS subtitles.
Plex clients on other streaming devices may also handle PGS subtitles (I have Android devices, so can’t say how others handle subtitles).
The Plex desktop apps, Plex for Windows/Mac, and Plex HTPC, also direct play PGS subtitles. Using one of those apps on a desktop/laptop connected to the TV may also be an option. Note that neither support displaying HDR video. It will be tonemapped to SDR by the application (not your server).
