I hope you're going to build only a sync plugin like suggested and not a whole clipboard manager.

Nobody needs yet another clipboard manager as there is almost a perfect one out there. Which can be extended by plugins! (I don't think it's a valid approach to start a new project because some project doing most of the wanted stuff isn't "perfect". The result is almost always "yet another 80% solution").

The "limited" number of entries is not an issue, imho. You can set it to x-thousand entries if you like. (One thousand entries are enough for more than two weeks of history for me. A clipboard is not a means of permanent data storage at all so this is fine; even one can have something akin to permanent entries by just using tabs in CopyQ).

Implementing in CopyQ some of the suggestions mentioned here would be great though!

I don't think a sync plugin would work. Entire parts of CopyQ's architecture would need to be redesigned, which would affect the rest of its ecosystem.

You can read the issue I filed in CopyQ's repository back when I switched to Linux and had to stop using Ditto for the justifications of infinite history: https://github.com/hluk/CopyQ/issues/510

CopyQ is also bound to use Qt's clipboard implementation, which provides the common subset of Qt's supported platforms, with all the consequent limitations. See the other issues I filed for details.