mac mini is not able to handle the LG 5k.
I got mine as a replacement for an always-docked 2016 MBP. Ran both back-to-back and the difference is clear. Even if youre willing to deal with some choppiness in Expose, Spaces, and other UI animations, large youtube videos drop frames and freeze on the mac mini. Scrolling webpages stutters. Theres also some minor bugs and glitches, i would guess driver bugs.
mac mini is base model i3/8G ram. MBP is 15" i7 radeon 455.
PS i thought about doing an egpu, but AFAIK the only TB3 egpu that can drive the LG 5k is the Blackmagic, which is almost 800$ with tax, and quite large (compared to either the mini alone or a docked laptop).