Yeah, ASS rendering was a big problem for me when I was making some online subtitling software. I ended up on using https://github.com/ThaUnknown/jassub + using Mediabunny to render a "subtitles" media track on top rendered on a canvas.

The result works pretty well, e.g. https://www.translate.mom/app/task/2UicdIqRBg0f