I don't know if anybody's interested in this, but I've successfully set up a COPS server on a Raspberry Pi2 with the help of DietPi OS.
DietPi made it very easy to add and manage Syncthing: through which I copy the calibre library from my PC to a USB stick attached to the Raspberry Pi.
I added the COPS server by following these instructions:
https://zer09er.blogspot.com/2020/04...cops-with.html
Together they gave me easy access to my e-book library via my Kobo e-reader's web browser.
While my Pi's calibre library is on a USB stick, if I was doing it again I'd probably keep things simpler by opting to store the library in the operating system's SD card. Though that would mean finding a much larger capacity SD card.
Hope this helps someone.