Hhmmm... The last time I looked at Readwise, I checked it's operation with Kobo and Kindle eInk ereaders.
Readwise did not support Kobo ereaders and checking
What highlight sources does Readwise support?, I still don't see Kobo listed as a source.
For my Kindle eInk device, Readwise would not sync any highlights for books not purchased from Amazon (this included books added to the Kindle using Download and transfer via USB) and using the import clipping file workaround was not very useful. I suspect due to the same issues that the calibre Annotations plugin has had with the MyClippings.txt. I tried both the email MyClippings.txt as an attachment and directly importing it.