Thinking on this a little more: say that you try Freda+ and like it.

Why not try the solution of...
* Run the Calibre server locally, same machine
* Use the feature in Freda that lets you connect to an OPDS server, and just take advantage of the metadata and book management scheme that you already created in Calibre? Freda can do that. Or if that doesn't work for some reason, Freda can also point to a website, which in this case would just be your Calibre server address.

One other misc thing: you don't have to 'script' file format association on Windows 10. Just go into the Default Apps settings, and near the bottom, there's an option "Choose default apps by file type." You can flip EPUB in there to use FB, Freda, or whatever you want. I just double-checked--mine shows Edge as the system default for Epub. But when I click it, all the other apps that can read Epub on my system, including Freda, pop up as options to set as the default.