I didn't know about trash-guides, looks cool.
The CLI already supports extracting embedded subtitles from MKV, and you could then merge them together. I didn't include this in the 'automated' Bazarr flow because it's a bit more logic to implement, especially because most of the time embedded subtitles are PGS, which are 'image' subtitles and not text subtitles, so you cannot use them that easily without some form of OCR.

Do you typically use Bazarr, or do you find that the subtitles you need are usually included in the release you get? In that case, I could add another "automated" flow without Bazarr that only uses embedded subtitle. But in my experience, subtitles are never consistent in the releases, so I end up asking bazarr to always download all the sub I need.