> Relatedly, there's an empty niche in the market for a USB-C power bank that can act as a UPS: able to charge and discharge at the same time without interrupting the discharge port when the charge port is disconnected

I think there's a soft version of this already available. The term is "pass-through" charging. The power bank that I have [1] will charge itself and the devices at the same time (albeit at reduced rates suitable for overnight charging).

I agree that it would be super useful to have a device that is explicitly designed to provide maybe 5V/5A for a Raspberry Pi 5 without interruption, and perhaps double as a power bank.

[1] https://www.amazon.com/dp/B0DHRYDNXL

As far as I know, pass-through power banks will still universally drop the power on their output momentarily when they lose the connection on their input, though. That's the missing bit.