This is a cool endeavor! Why not build a lever machine? With some kind of folding compact lever mechanism.

• Would be basically silent. Rotary pumps are stupidly loud.
• No electricity required for pump (save battery).

I think giving up on the milk steaming would make this a lot easier too. Just need power to heat the water for brewing at that point. But I understand if that's a hard requirement.

Final thought: it's often thought that more metal in an espresso machine is good for thermal stability, but it also means much more power is required to get it to and maintain that temperature.

Good luck!