To me it sounds like reenabling the fallback with trying TLS1.2 after TLS1.3 fails for a connection would be the best solution to gradually upgrade all devices.
TLS is already so complex and ASN.1 makes xml look like c-struct. This begs the question: why would anyone want to make a crypto overly complex ?
If fallback works, fallback works. End of story. 80% of operators (governments, corporations, soho setups) will explicitly prefer not touching things if at all possible. They won't fix their "not broken" network.