The suggestion to fallback on TCP feels like a hack.
It is not a hack, it is in the specs, see https://datatracker.ietf.org/doc/html/rfc3261#section-18.1.1:
``` 18.1.1 Sending Requests
The client side of the transport layer is responsible for sending the request and receiving responses. The user of the transport layer passes the client transport the request, an IP address, port, transport, and possibly TTL for multicast destinations.
If a request is within 200 bytes of the path MTU, or if it is larger than 1300 bytes and the path MTU is unknown, the request MUST be sent using an RFC 2914 congestion controlled transport protocol, such as TCP. ```
If someone wants to implement your suggestions, they are more than welcome, but it should be controlled by a parameter.
Should nobody commit to implement this feature request in one month or so, the track item may be closed.