This is what I'm currently doing sans tailscale. I'm running Ubooquity on a server in my homelab as my OPDS service to serve the ebooks hosted on a mounted NAS. I can download any of those books from my Kobo with a few presses on Koreader. It's pretty great. My Kobo Forma is probably one of my best and most used tech purchases. I've had it since 2019 and couldn't be happier with the device + setup. Getting it set up with tailscale so I can fetch ebooks when I'm away from home sounds like a pretty good upgrade.
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.