For me the big advantage was just never having to think about sync conflicts ever again, which happened to me often enough to be annoying. There's also things like the browser integration is more streamlined, using the bitwarden CLI is convenient in some of my automation, and while android supports syncthing, android has always been a bit less reliable in desktop platforms in allowing syncthing to run in the background or in allowing other apps to know that syncthing has updated a file.

I use Dropbox for my mobile devices and keypassxc had been great. I had the same issues as you with syncyhing on mobile.

I actually moved from Dropbox to Syncthing as I found Syncthing more reliable, as Dropbox would sometimes generate spurious conflicts even between my desktop devices.