I've just looked at the OpenSSH source, and I agree it should be configurable. That seems like an easy patch if you wanted to do it.

Or, better but more difficult, it should track the dynamic TCP window size, from the OS when possible, combined with end-to-end measurements, and ensure the SSH mux channel windows grow to accomodate the TCP window, without growing so much they starve other channels.

To your second point, you can't do data dropping and retransmission for mux'd channels over a single TCP connection. After data is sent from the application to the kernel socket, it can't be removed from the TCP transmission queue, will be retransmitted by the kernel socket as often as needed, and will reach the destination eventually, provided the TCP connection as a whole survives.

You can do mux'd data dropping and retransmission over a single UDP connection, but that's basically what QUIC is.