The Mac mini is very tempting, but only if all major software works with eGPUs: FinalCut, Adobe, etc. By the time you max out a Mac mini, buy the BlackMagic eGPU, and a 5K monitor, the cost is way above even an iMac Pro. I think the 5K iMac is a sweet deal when it finally gets updated, hopefully in 3-4 months time. The only disadvantage of the iMac is that it cannot be used as an external display for other machines. It's not upgradeable, but it's the least amount of hassle overall, and you can sell it when you're done with it, because it's so standard. Try selling a Hackintosh. Enthusiasts always build their own, and businesses need something standard, something they can buy 10 identical copies of, with support and all that stuff.
There's one way a Mac mini works out: If you're going to use the external monitor with multiple machines. Let's say you can connect a Windows or Linux machine to it, a laptop, a MacBook, even an iPad Pro. You can upgrade all of your computers, and keep using the same display.
Also a 4K display may be enough for a lot of people, which is much more affordable than a 5K one. But if you want an amazing 5K display with a pretty reasonable computer, the iMac seems to be a good deal.