A combination of the answers in here + killing all QuickLook-related processes fixed it for me. I have been experiencing this ever since the Mojave update. No reboot required.

qlmanage -r commands always got stuck for me, too.

Fix TL;DR version:

1. Delete DropboxQL.qlgenerator from ~/Library/QuickLook (Credit to suntrop)
2. Restart Finder
3. sudo update_dyld_shared_cache -force (Credit to Dishoperator)
4. Kill all QuickLook-related processes
5. Restart Finder

Fix detailed how-to version follows

For those not very familiar with what the above means.

To get QuickLook working when previewing files with spacebar:

1. Delete DropboxQL.qlgenerator in ~/Library/QuickLook, as per suntrop's suggestion.

   • Open Finder
   • Press ⌘+G
   • ~/Library/QuickLook
   • Look for a file named DropboxQL.qlgenerator, it will probably be the only one
   • Delete it, and then delete it from Trash

2. Restart Finder:

   • Press ⌘ ⌥ ␛ (Command + Option + Escape)
   • Find Finder
   • Click Relaunch

To get thumbnails to work:

1. Run the Terminal command Dishoperator suggested: sudo update_dyld_shared_cache -force

   • It doesn't do any harm, it just updates the kernel caches
   • Open Terminal
   • Paste sudo update_dyld_shared_cache -force
   • Type in your password (it will not appear as you're typing)
   • Press Return
   • It will take ~30s to a couple minutes depending on your machine

2. Open up Activity Monitor (⌘+Spacebar, type in Activity Monitor)

3. Order by Process Name

4. Force quit quicklookd

   • Click on quicklookd
   • Press the X stop-like button in the top left
   • Press Force quit

5. Force quit all instances of QuickLookSatellite

6. Keep force-quitting until none of them are left

   • Make sure you order processes by name to see them all at once

7. Restart Finder (see point 2. Restart Finder: again if you forgot how)

Keep having problems?

Since this is now working for me, but in case it still doesn't for you, here are some apps that would normally be suspects but are NOT the cause. This should narrow-down your search:

• Dropbox (was a problem)
• Sketch 52.2
• Principle
• CraftManager
• Photoshop CS6
• Illustrator CS6
• Bear
• Highland 2
• Kindle
• Atom, PhpStorm, Sublime Text, Caret
• p4merge
• Slack
• Screenhero (don't know why I still keep it)
• SourceTree
• Chrome, Safari, FireFox
• Franz
• 1Password
• Spotify
• Shazam
• TeamViewer
• Steam
• Flux (don't know why I still keep this one either)
• Microsoft OneNote
• Bartender 3
• PSequel
• Postico
• GoToMeeting
• Kaleidoscope
• MindNode 5
• Podio
• Fluid UI
• Gliffy Diagrams
• Conceptboard
• Moqups
• Wireframe.cc
• Graph.tk
• SketchUp, Layout, StyleBuilder
• Google Keep
• VLC
• Trello
• Asana
• LiveMinutes
• Zoom
• Amphetamine

And remember to clean up useless apps from your laptop. Which I clearly don't.

How could updating to the latest OS have caused this to have happened or could it just be a coincidence?

It's entirely a coincidence. Many times, people incorrectly correlate an activity they are doing at the time with failure of an unrelated component. You could have been doing a Time Machine backup, just the same, it it would have failed.

I haven't tried reformatting it yet but could it help?

You don't need to. You can boot into Internet Recovery which is the same as booting into a "fresh" install of macOS. You can also install macOS onto a flash disk and boot from there to test. There's no need to reinstall over your existing data to see if you can fix your display.

The guys at the genius bar informed me that there is an issue with the display and they will need to replace the whole top of the Mac

Yes, you definitely have an issue with your display. However, I'm not convinced that it's your screen as it could be your GPU, backlight or the LVDS connector/cable. I would take it in to a different, reputable shop for a second opinion. See this video for additional information.