Reading SD cards on 2018 Mac Mini

Yes it will work on the Mac mini.

I bought a cheap, diminutive hub (SD card + uSD card slots, plus 3 USB 3.0 ports) from Ali Express for $US 3.37 shipped that uses one of the USB-C ports. This helps with there only being only two USB 3.0 ports (which I use for two USB drives because of the USB-C power drop issue).

So, one USB-C is used by the VGA adapter and another by the hub, leaving two free USB-C ports and the three extra USB 3.0 ports one of which is used for an external speaker and one for a PIC programmer, leaving one USB 3.0 port free.

I haven't had any problem with the mini's 5G WiFi nor my wireless MS mouse.

Yes but a simple USB card reader is better as hubs can interfere with WiFi. What other ports do you need? The mini has 6 USB ports. For most users a hub is unnecessary.