Awesome

rePub

A reMarkable ePub generator. This is essentially an open source version of Read on reMarkable. In contast to that extension, this will include images in the generated ePub files. It originally offered more configuration options over the original extension, but those are disabled until a new API can be found.

However this doesn't replicate the printer adapter, so if you want to upload PDFs it still recommended to keep the Read on reMarkable extension for those uploads.

Firefox

Currently there's an unaffiliated hard fork that may suit your needs. Full support is planned