On-orbit refuelling at such a scale does sound rather difficult, but they also fully intend to do it in the near future. They're certainly going to give it a good try.

Also, if you don't want to make an entirely different second stage to put on top of superheavy, you should be able to fit a pretty substantial third stage in Starship's payload bay.

Right now the payload bay doesn't open anywhere near enough for that, but hopefully in the future that'll be an option.

And if you really want to make something go fast, you could refuel in orbit, give it a push from Starship, then deploy the third stage for another kick.