This add-on seems to be a really good effort. Unfortunately there are a few issues:
1. Most importantly, every time you pause or jump to a different timestamp it takes 5-10 seconds until it displays subtitles again, which is quite a dealbreaker for me.
2. The options menu doesn't stay visible (using Amazon Prime), even with the mouse cursor hovering over it. And somewhat funnily it will only show up again if the cursor leaves it.
3. A significant amount of time when looking for subtitles with the built-in search it will just show an error after loading for a while. I realize that this is possibly due to the external provider, but even then the error handling is suboptimal, since it throws you back to the main menu afterwards, meaning the whole process of searching and choosing a subtitle has to be done over and over again until it works.

Additionally, as I just noticed the timing seems to be off from the file. In my example the first line in the srt file starts at 00:00:19 seconds but after loading it into the add-on it shows the first line at about 00:00:09. Synchronization option is set to 0.

Also, having the option to add outlines and/or shadows to the subtitle display would be beneficial for visibility.