Comment by Larrikin

The setup is

install Tailscale on your Kobo

install Koreader

Install Tailscale on the machine that host your eBook collection app of choice

Add the OPDS URL from the collection app, replacing the local machine URL with the Tailscale URL

You can now browse and download your private collection from anywhere.

I went with Kavita since I wanted my eBooks treated as equals with my manga.