I think CaseySJ has warnings about Thunderbolt 3 monitors. You should see that somewhere if you search this thread. Displayport and HDMI monitors work pretty much without fuss though.
I'm running the same build as Casey for the last few years, & I'm using the following.
(Still on the original clover that was in the guide here and on Mojave.)
1. DisplayPort out (from RX 580) to Display Port in (Gigabyte motherboard/ intel IGP).
2. Built in gfx USB-c/Tthunderbolt out to LG 32" UN880-B Ergo monitor from CostCo. (It's a superb monitor.)
3. BIOS will not display during boot up or Clover, but after a few seconds you will see the Apple logo & the system will start up fine.
You can switch to DP if you really need access to Clover, etc.
My main issue on this setup is the on-board USB ports on the back of the monitor do not work.
I'm using the USB-c cable that came with the LG monitor.
Now, I know for sure that the ports on the back do work if I unplug the desktop & use my wife's 2019 MacBook Pro & plug that into the USB-c.