About the wizard, it would help if you just had a "Subtitles" checkbox somewhere which then adds a blank <track> line to the script.

As for the synthax question, my point is that your script looks like this

  <video
    slot="media"
    src="https://stream.mux.com/fXNzVtmtWuyz00xnSrJg4OJH6PyNo6D02UzmgeKGkP5YQ/low.mp4"
    playsinline
    crossorigin
  ></video>

And media chorme looks like this

<video slot="media" src="./video.mp4" crossOrigin playsInline>

<track label="English" kind="captions" srcLang="en" src="./captions.vtt"></track>

<track label="thumbnails" default kind="metadata" src="./thumbnails.vtt"></track>

</video>

And I just don't know how to interpolate the two

(Edit, checking further, I did manage to mix the two, and I can play subtitles over your demo video, but not over mine so I guess a foolproof sample in the wizard is probably needed :D )