So in case someone else is annoyed too and doesn't want to use the website version: I have set up a simple Telegram bot that uses Kepubify v4 to convert. Just send the epub in the chat to get a kepub back:
https://t.me/epub_kepub_bot
I wrote it mainly for myself, but maybe others will find it useful too. It is meant for single books from time to time, please don't blow up my traffic by uploading your entire library. I have set a limit of 5 conversions per user per hour for this reason, so that the bot can stay online as long as possible.
This code is used