$799 for the base Mac mini, with 8GB RAM and 128GB SSD is overpriced, and it just gets worse from there. Upgrading to 16GB is $200; you can get a 8GB stick for under $50, and a 16GB stick is around $100.
1TB is an $800 add-on from Apple. A top of the line, 1TB Samsung 970 Pro is $350, and you can find slower (but still very fast) 1TB NVMe SSDs for $150-250. A 2TB NVMe can be had for $550 or less; Apple wants $1,600 extra for that.
Move away from NVMe, and you can get 2TB for under $300. But of course a 2.5 inch SSD won't fit into the Mac mini, because Apple focuses so much on making it, well, mini. But then to add storage at an affordable price, you've got to go to external storage, as you suggested. At this point, why do I care that the mini is mini? I'm not saving any space on my desk, and now I've got an additional tangle of cords. And so much for the speed advantages of NVMe.
And what an odd pairing high performance NVMe SSD is with low-end, integrated graphics. Want better performance? Throw an external GPU on your desk and credit card. Now your computer isn't looking so mini in size or price.
The Mac mini reflects the worst of Apple right now. Too expensive to be entry level, too limited to be high end. Add external devices to overcome the limits, and its compactness is no longer an advantage. What is the niche Apple is trying to fill here?
I fear the next Mac Pro will take everything wrong with the mini and crank it up a few notches. We don't need an "modular" Mac; we need an upgradeable one. There's a simple elegance to being able to easily add memory and storage, and to keep everything neatly contained inside a single box, so you don't have a tangle of cords and devices. I could do that with my PowerMac towers. Even my 2009 iMac I was able to upgrade with an SSD and an internal hard drive (I removed the DVD).
As I said before, this doesn't need to be complicated, but Apple insists on making it so. Apple's hubris is to blame. They think they must "innovate" for the sake of being different when tried-and-true solutions are not just less expensive, but superior. And they are looking for ways to milk their customers. Why allow people to install their own RAM for $50 when we can charge them $200? Why allow them to upgrade later when we can sell them a new machine (if we ever get around to releasing a new machine)?
I'm not going to say, that's it, I'm moving to Windows. I won't. I hate Windows. I love macOS. I used to love Apple. Which is why it's so frustrating to see them with their heads up their you-know-what. The irony is that they'd produce better machines if they tried a little less hard. If I sound frustrated, it's because I am. It's hard to believe that such a group of smart, talented people can be so stupid.