The problem with ssh is that it's a live connection; rubbish over a slow or packet loss full connection like satelite.

Surely there's got to be something out there that allows you to compose the command you want and then submit it in one go with the return key?

Yes:

I could combine that with passwordless logins using certificates but I don't see how that's much quicker than preparing the command in a text editor and then copy/pasting it, which is what I have to do at the moment. You also loose tab completion with that.

Let's say it was windows and PuTTY as an example so it's clearer what I mean. It would be better if there was a box below the window that automated copy and paste by being able to type locally in the lower window, and if you press tab it requests that in a separate thread, then when you press return it pastes the command into the live window.

Tab completion is a function of the ssh connection being live and responding to user input in real time. I can't see how it would be possible to have both an offline entry (for a slow connection) and auto completion. If you have to deal with the slow connection using the ssh -t option to send commands works, as would cut and paste, or just fighting and typing in live (depending on how much needs to be done).

Good luck finding something that allows the best of both, and if you do I will be sure to keep it in mind for myself.