This breaks rsync & many other things that rely on SSH as transport. Try:

  alias ssh="ssh -o RequestTTY=force -o RemoteCommand='tmux new -A -s foobar'"

I use an ssh virtual host pattern *-tmux to copy over an ssh-specific tmux.conf to a remote host before connect and then connect to it with -f tmux-ssh.conf. I can also connect to a host directly without tmux stuff by omitting the -tmux suffix.

Or just use a different myhost, like "t-myhost" (i.e. myhost != hostname)?

Which would require creating a couple of hundred (in my case) duplicate host aliases instead of adding one shell alias that can be used with any host, including those not mentioned in ~/.ssh/config.

You can resolve this with a conditional. See the `Match` keyword.

Though I agree, probably not the best solution to start all sessions with tmux. There's other issues it can cause.

No, host matches allow wildcards.

Please don't do this. Do something like instead except don't call it .tmux.conf, use .tmux-ssh.conf and -f: https://stackoverflow.com/a/66709528