Hi Laura,

> I find a solution for not getting the whole text of the subtitle at
> once, that is creating each svg/cli session/whatever
> appears-in-the-video-if-needed with their audio and subtitle
> separately and glue every piece of video as a last step with ffmpeg.
> Apparently, that is possible according to
> https://trac.ffmpeg.org/wiki/Concatenate - have not tried it yet.
> However, I don't know if, in case of wanting to have the subtitles
> separately - not  hardsubs-  in this latter case if it is possible to
> concatenate them and use them outside the video.

I've done this many times and it works very well.  Off the top of my head,
something like

--8<---------------cut here---------------start------------->8---
  ffmpeg -i movie.mkv -i sub1.usf -i sub2.usf -map 0 -c:s copy -c:v copy -c:a copy output.mkv
--8<---------------cut here---------------end--------------->8---

should cut it.

I would not recommend hard-subbing.  It's less flexible and does not allow for
multiple languages.  Instead, you can use the .mkv format which allows for
embedding multiple subtitles, choosing a default subtitle, and finally leaves it
to the user to disable them when playing.

Cheers!

-- 
Pierre Neidhardt
https://ambrevar.xyz/