I just tested using Seedvault with Syncthing and to me it feels like a great setup. I still need to configure it for all time use, but so far, first test seems like everything works fine. It allows me to use a connected service where my data isn't just one place, without needing physical things like a USB flash drive, etc, but without needing to pay for the cloud, and dealing with bad speeds, especially with something that can be as demanding as device backups.
- Your device essentially has half the storage, since you'll need to keep your backups inside your phone, so there's redundant data there.
- This is not a fully tested setup, and you may lose data with it.