Crazy how that doesn’t at all even address the problem of subtitle sync!

As I said, this isn't even an issue with Jellyfin. It's an issue with the device that's playing the media--your television (or chromecast). This workaround makes an exact copy of the internal subs, and dumps them to an SRT which allows your television (or chromecast) to play the internal subtitles as external subtitles...

It has nothing to do with subsync, it's not syncing subs. There are no "mistakes" because you're pulling the internal subs exactly as they are internally, externally...