Will this Mac mini setup work as I intend?
The 2018 Mac mini can output two displays at 4096 x 2304 from its Thunderbolt connectors, and one 4096 x 2160 display via its HDMI port.
So a single 3840 x 2160 display is well within its capabilities.
The mini has 2 USB A ports, but 4 Thunderbolt ports, which also provide USB C. You could use a USB C to A adaptor or cable to connect a USB device.
I have a mini with an Anker 7-port USB hub, and that powers both my iPhone and external USB drives. My iPad, however, does not charge through the hub. Keyboard and mouse take minimal power. You might want to connect the drive to the mini directly, and the hub to the 2nd USB A port.
Most hubs do come with an optional power supply, but I would expect that would only be needed for multiple high-power devices.