Using SSH ForwardAgent and ProxyJump with IdentityFile in command line
Solution 1:
You can't do that with the new jump magic as far as I know. But it should work with the "old" proxy-command:
ssh -i ~/.ssh/mykey.id_rsa -Ao ProxyCommand="ssh -i ~/.ssh/mykey.id_rsa -W %h:%p -p 1234 [email protected]" -p 5678 [email protected]