I use this monitor, and really like it.
LG 27UD58-B
The 27” size is very nice for me, and I wouldn’t adhere to any hard and fast rules about what should be 4K-sized, and what should be 5K-sized.
This size and resolution lets me have a PDF open on one side of the screen, and a document or another PDF open on the other side.
I use Magnet to quickly size windows to different halves or thirds of the screen.
This monitor will also allow you to use it with two different computers, if you have the need for that. For instance, you could have your Mini on one side, and a laptop on the other side.