Easiest is with KOReader. You start a server in Calibre then connect to Calibre in KOReader (not via the web browser). At that point it's like you plugged it in via USB, you can add/remove from the device from your library.

I have my Calibre install setup with the content server feature which makes your Calibre library accessible to any device on the local network. I can then transfer the books to my Kobo by using the browser and hitting the content server's IP address.

I can't remember all the specifics of how I set it up and honestly i've not used it in a while because I used it to get all my old Kindle purchased books on (which was a lot) and i've not booted the docker container since.

Is this a reasonable work around/alternative or have I misunderstood? If so, I apologise. Oh also, I think I had to do some custom stuff with my Kobo as well to support this functionality, but I don't remember it being difficult. Got it all wrapped in an evening of tinkering.

Try this: https://send.djazz.se/

Minimal work if you configure the site as the homepage in the Kobo web browser.

You can use Google Drive with Kobo.