I have a 21.5-inch UltraFine paired with my 2018 base model Mac mini. I managed to get a brand-new boxed unit off of Ebay about three months ago for half the original MSRP. Where this technological leprachaun had been hiding for over two years, I'm not sure, but it was sealed at the factory. So, it is still possible, if very difficult, to find an unopened unit. Still, if you can find a used one that has no defects, then that would be my recommendation.
Having tried many monitors with Macs, with various ppi densities, I must say that I absolutely prefer the 219 ppi that Apple recommends. Everything else falls short, particularly with the way macOS renders to high-dpi displays. Having experienced it for a few months now, I personally would only go with a display that Apple would qualify as "Retina". Unfortunately, that narrows the choices down to the discontinued 22MD4KA, the UltraFine 5K which is currently having availability issues, the professionally expensive XDR, and of course the iMac line if we include entire desktops.
It depends on your priorities, monitor availability, budget, and personal preferences for a display. I personally would do my best to go with a monitor within Apple's recommended guidelines.