I know your pain, running two monitors off my 2010.
A DisplayLink box is our only choice and works pretty well through High Sierra OS 10.13.3.
With 10.13.4–6, Apple disabled eGPU support except for TB3 and that disabled DisplayLink, too. Apple re-enabled DL support in Mojave but 10.14.x is useless on our machines.
There is a kludge involving Air Play but (surprise, surprise) that isn’t available on the iMac till 2011.
https://www.displaylink.com/downloads/macos
I don’t know if you can revert back to 10.13.3. It will be a time consuming PIA unless you are on a large SSD formatted APFS and have a lot of room and APFS Snapshots pre-dating 10.13.4—then it’s easy. If you had done this within a few days, it would have been dead simple — so easy that I stumbled onto it accidentally:
http://www.motunation.com/forum/viewtopic.php?f=4&t=64689
If you’re on an HDD or didn’t format your SSD to APFS, the above cannot work.
Otherwise, you have three choices:
1) Save all work product since you went to 10.13.4. Wipe your drive. Install 10.13.3 and restore from Time Machine or other backup that predates your 10.13.4 update. Reload your last year’s work files—easy from a Time Machine backup if you had allowed Spotlight to index your TM drive.
2) As above but do a clean install of your applications after you are running 10.13.3. You can then pull your work files (Documents, Photos etc.) from a Time Machine backup or clone. I did that in 2013 and it took three days.
3) It’s time for a 2011 or later iMac with two TB ports. A 2011 will run 10.13.6 — a pair of inexpensive MDP adapters will get you hooked up without the need for your DisplayLink box.
For (1) or (2), you’ll need the 10.13.3 complete installer. AFIK, you won’t find that anywhere online but I have it and can get it to you through GoogleDrive, FTP or Dropbox if your account is large enough for a 5.8GB file. Apple still makes the 10.13.6 installer available but that won’t help you.
I chose option (3) and am loving my iMac Pro. Anybody need a used DisplayLink box?
[doublepost=1556712781][/doublepost]If you are running certain apps, you will find a clean install impossible. The apps still run in High Sierra but their installers don’t. Office 2008 comes to mind.
The workaround is to make a Yosemite install USB drive, erase, reformat your and install OS 10.10. All your installers should run. Now use the 10.13.3 complete installer to update the system.
Again, you should consider getting a 2011 or newer iMac.