Get thee to MacPro..... What you wish for is not a Mac Mini
A snip at only 6 times the price... including a huge premium for a Xeon processor (which you probably don't need) ECC RAM (which you probably don't need) and an insane number of PCIe slots (8 is probably 6 more than you actually need)... and if you do need that sort of expansion capacity, you probably need something better than the "only" 8-core CPU in the entry model.
Oh - and you can't actually buy it yet (past halfway through October).
Even the previous Mac Pros (cheesegrater and trashcan) were a more credible "next step up" from the Mini. The new Mac Pro doubled the entry price.
I think what he actually wishes for (as do so many people here) is the mythic headless iMac a.k.a. xMac or non-pro Mac Desktop.
Given that Apple aren't likely to do that, what
some people want is a Mac Mini with even a half-decent
mobile GPU that could comfortably drive a pair of 4k displays in Mac OS "scaled mode". I mean, its a brilliant bit of marketing by Apple:
save money by switching from expensive mobile chips to cheaper desktop chips with lowest-common-denominator, good-enough-for-spreadsheets iGPUs, then claim it as an 'upgrade' that justifies a price rise. C.f. the MacBook range where, in the past, they've held the CPU back a generation in order to hang on to Iris Pro/Plus/whatever graphics.
I'm surely keeping an eye on the second-hand market to possibly replace my 2012 mini at some point in the future (though I may as well switch to a simple RasPi when the mini should eventually fail).
If you don't need MacOS, there are a host of small-form-factor PCs to choose from - or you can build your own using a Mini-ITX board (or one of the even smaller options). Hopefully, soon, there will be some well-supported ARM options apart from the Pi (the Pi is unbeatable at the price - and the Pi 4 has improved the USB 3/network I/O considerably - but without M.2 or SATA its not really credible as - and was never really intended as - a PC/Mac replacement.