> None that I know of is designed to operate from a single Thunderbolt cable like a docked laptop can.

The Mac Mini is basically exactly this, with one exception, which is power. I really wish this existed as well. An M2 Mac Mini that supported being powered over TB with PD, would be perfect for me, because I have multiple work locations that each have monitors that provide 90W of PD power over TB, and I could simply drop it in my bag.