Not 5K but I’ve been using a 27" Apple Cinema display for 8 years now.
I use Moom for window management. I hear Spectacle and Magnet are good too. Is there any difference between them, other than pricing?
Most of the time - like right now - I run one app at a time, maximized to the full screen. But not in actual fullscreen mode, because I like to be able to see the menu bar, with the clock and iStat Menus and various menu bar icons showing.
When I’m writing, I have my text editor occupying half the screen. The other half the screen is occupied by a source document - notes, PDF, or a browser window. I use Moom to handle window management.