There’s one thing I don’t get about the complaints that the new Mac mini isn’t expandable: Have you already forgotten Apple’s strategy with the Pro [before we knew it was a thermal disaster]? Thanks to Thunderbolt 3, the expansion is on the outside, okay?
Get a new mini. Upgrade the processor to the “3.2GHz 6‑core 8th‑generation Intel Core i7”. Click “Add to Bag”. Pay $1100 and enjoy.
You have plenty of SSD to run the OS. Buy a fast Thunderbolt 3 external SSD if you need more space. Heck, buy a fast TB3 drive if your internal SSD ever dies and put the OS there.
I’d be surprised if the external hard drive ever noticeably slows you down. You can put your video card outside of the mini, for heaven’s sake, and I don’t hear anybody complaining about that – other than the price for the enclosure. But they don’t complain [much] about the speed.
Seriously. This is the mini you said you wanted. Enjoy it. ;^)