For iOS devices the most up to date client is “Strongbox”. I don’t think it is FOSS, but is compliant with the standard. It’s sadly a freemium app, but is quite well made in my testing. It cannot sync with syncthing, but does support several cloud services, its own service (which uses iCloud), and local file transfer over LAN. They also have a version of the app with all network connectivity removed for security (if you prefer)
If you don’t mind me asking, what makes gnome more adaptable in terms of functionality than KDE?