[JawadLeLogeur]

The waterproof and physical button requirements means that you've pretty much already found the only two possible choices to the best of my knowledge.

Pocket is a bit hit or miss on Kobo, but it does work (not with a Firefox account however). Since you mentioned KOReader, you can also switch to Wallabag from Pocket which is supported on there (I'm not affiliated with either KOReader or Wallabag but it's what I use daily) however I would try it a bit first before committing since the Wallabag integration might not be for everyone.

For color PDFs, just be aware that e-ink colors look a bit washed out, similar to what you'd find on newspaper cartoon prints if that makes sense. Still amazing, but a factor to consider.

I don't think you'll be able to play videos well, for that you will either need one of those Android readers from various PRC companies such as Onyx Boox (none of which are waterproof AFAIK) which support high refresh rates or invest in a reflective LCD tablet. Non-Android eink readers are pretty bad for web-browsing in general, if not lacking the feature altogether.

Ultimately it will come down to whether you prefer listening on speaker or with bluetooth headphones.