In theory you could set up a simple sync server that implements the necessary endpoints and nothing more, nothing less. Guides like these run the official, Mozilla-maintained sync server, which is obviously based on serving millions of users rather than being a minimal implementation for someone in a home lab.
One could fork https://github.com/mozilla-services/syncstorage-rs, take out the plumbing, and reimplement it all without a secondary database and the Google Cloud base architecture, I'm sure, but like with so many things, you'd first need to gather a group of people with enough interest and spare time to do the development.