I see Apple are still advertising the LG UltraFine 5k (albeit with 4-6 week delivery) which, basically, is the iMac display in a boring black plastic case (...but with a better stand). But if the 27" 5k display is what you really want, it is hard to touch the iMac for value.
Problem is, 5k is the sweet spot for MacOS which doesn't really have a variable-PPI user interface (& of course 5k is twice-1440p as used in older iMacs and cinema displays). But Windows (a) is more-or-less optimised for 1080p and (b) does have adjustable PPI so 4k pretty much hits the spot. 5k displays did appear but pretty much fizzled - even the LG UltraFine seems to have one foot in the grave.
Frankly, the difference between 5k and 4k isn't that huge but for the fact that the Mac needs to do non-integer scaling and re-sampling to get the UI elements to a sensible size for a 27" 4k screen.
If I were going for a Mac Mini I'd "think different" about displays: maybe a pair of 4k 24" displays, or a large 4k TV/display that was big enough to use at native resolution....?