As you might notice CC has a new owner and somewhat uncertain future on Android.
I found
Calibre Sync. This is an abandoned project from 2013/2014 which does implement two of the major features of CC: the wireless device client and the opds client.
I tried to
contact the developer showing my intent of updating it and releasing it on F-Droid.
Until the developer gives feedback I cannot release a binary, but I updated the project to work with latest android studio tools and latest sdk and is better than I thought.
It has some bugs and some memory leaks, uses some deprecated apis and it is pretty limited presenting information to users. But for a 2013 program it seems really well designed and after bumping some things I got it working ok in android 10 emulator.
Now I'm looking for other people interested in contributing with the project (or another if the copyright holder denies us access to its source). There are a few things that need work.
If you're interested (as helper or developer) please send me a private message and we can talk.
I'm not really a hardcore calibre user nor a hardcore CC user and this has lower priority for me than other hobbies, but I would love to see a opensource alternative reaching stability before CC is turned into a subscription model business.