As mentioned before, October should work fine with ePubs but Kobo has its own variation called Kepub.
Regardless of what format you pick, it's recommended to upload books via Calibre (and fetching metadata) to ensure that your Readwise shelf entries have as much detail as possible.
There is a lot of detail involved in comparing the two but in short, I can personally recommend using Kepubs if you want to enable extra features such as:
- Page turn requests being quicker
- Highlighting text is faster
- Ability to see how many minutes of reading are left for each chapter
For our purposes, highlighting is core to our use case when it comes to Readwise so it's worth adopting but there is a bit of overhead required to get started.