Separate export for FORCED and regular SUBS

[Woodstock]

MP4 from a DVD source with VOBSUB subtitles, that's a viable option.

BD source, you can't have switchable subtitles without converting the PGS to something else. VOBSUB can work (I used to merge the DVD subtitles on dual-media sets to the BD video/audio). SRT or SSA can work, if you get the conversion right (or use an external subtitle track).

Subtitle Edit says it can "compare subtitles" on their feature page... then doesn't expand upon that.

Today I re-watched an anime that had Japanese subtitles hard-coded in it, and dual English tracks. Subtitles can be VERY weird!

[DJRumpy]

Your best bet with Plex, and MP4 or MKV containers, encoded with Handbrake, are SRT, hence the ask.

[Chetwood]

I'm encoding them to H.265, burning in the forced at that time, but leaving the non-forced as user selectable.

Which could be done just as easily with one forced track and one regular track, both not burned in. Even if you insist on MP4 as a container, most players would play the external subs just fine so again no need to OCR to SRT. YMMV depening on the player of course, Plex apparently does not feature PGS support on all clients without transcoding.