I'm not sure the specification says what you describe, it doesn't in my reading.

I don't believe Alt-Svc is required, and as you say Safari is behaving this way already.

https://www.rfc-editor.org/rfc/rfc9114.html#section-3.1-3

Discovering an HTTP/3 Endpoint

A client MAY attempt access to a resource with an "https" URI by resolving the host identifier to an IP address, establishing a QUIC connection to that address on the indicated port (including validation of the server certificate as described above), and sending an HTTP/3 request message targeting the URI to the server over that secured connection. Unless some other mechanism is used to select HTTP/3, the token "h3" is used in the Application-Layer Protocol Negotiation (ALPN; see [RFC7301]) extension during the TLS handshake.

Connectivity problems (e.g., blocking UDP) can result in a failure to establish a QUIC connection; clients SHOULD attempt to use TCP-based versions of HTTP in this case.