TLDR: XFCE and Cinnamon devs are begging beginning to work on Wayland support.
Yeah, as mentioned earlier: that is an application that directly works with X to interact with the mouse cursor. It needs to be updated or rewritten. No alternative to that, I’m afraid.
Check the input leap project. While I haven’t tested it myself, Wayland support got added like a year ago. You still needed to rebuild some packages, but reading the issue tracker now it seems to have gone a long way.
Unfortunately it is still not considered production ready. At this point I assume they will have it implemented and ready way before synergy though.