I use a mix of GSConnect/KDEConnect, Warpinator, and Syncthing. I’ve got a shared “dropoff” folder on Syncthing that lets me easily drop files from one device to another. You’re having issues with Warpinator but if you’re able to figure out the issue there, that’s my second go-to for one-time file transfers. KDEConnect is a bit more fiddly, but I use it mostly for sharing clipboard info and the occasional file when it’s stable enough.