Couple of things...also new mac mini 2018. Same bluetooth mouse problem with a USB 3.0 hub plugged in. Got an external bluetooth dongle to work, and that fixed the bluetooth mouse problem, but haven't figured out how to make the external dongle the default bluetooth controller yet. So I'm not using the USB hub...bummer.
On monitor cables, the cable that I had that was a new mini displayport to USB-C/Thunderbolt 3 cable, and it worked for about a week and a half, and just stopped working. So I figured out how I could use an HDMI cable, tried a random HDMI cable that I had lying around, and that's been 100% since.
On disk space, the mac mini that I have is only 256GB SSD. So I added a 500GB external SSD, a Samsung X5 (NVMe), runs like the wind *smile*. Very fast Thunderbolt 3 SSD, happy with it. Not cheap, but less expensive than having Apple put in a larger SSD. And I expect to see those getting cheaper as time goes on. Definitely enough fast storage for the time being.
I'm really happy with the mac mini aside from not being able to use a USB 3.0 hub. Still working on that one.