I have always used SRT subtitles when using English Forced Subs. Have never had any problem with them. Now that I’m moving to more and more 4K material in MKV, TS, and M2TS formats, please advise whether these subtitles will NEED to be removed and added externally to the folder to eliminate the possibility of recoding.
From what I’ve seen, it doesn’t make a difference if a subtitle is in an external file or embedded in the movie file itself. If the player can deal with the combination of your source materials, it’ll play directly – if it can’t, it’ll transcode independent if the subtitle is internal or external.
if needed (not for Plex… but for whatever reason) – you should be able to do this using a basic script to run through all files of a folder and process the files with MKVToolNix (e.g. using mkvextract from their set of apps). https://mkvtoolnix.download/doc/mkvextract.html