Just completed the upgrade to Catalina with a 2019 LG 5k 27MD5KL a few differences from Mojave.
Gigabyte Aorus Pro Wifi
Intel core i9
Vega 64 8GB
SMB iMac19,1
Display 1 LG 5K (Summer 2019) - Gigabyte GC-Titan Ridge - 2 DP in from the Vega 64 with 1 Thunderbolt (Type C) out going to the LG 5k. The GC-Titan Ridge connects to the thunderbolt header ion my motherboard and the display audio and webcam work out the box.
Display 2 Apple Thunderbolt 27” - Gigabyte GC-Alpine Ridge - 1 DP in from the Vega 64 with 1 Thunderbolt (Type C) going to a Thunderbolt 3 to Thunderbolt 2 adapter going to a 27” Apple Thunderbolt Display. The GC-Alpine Ridge in plugged directly into my motherboard using a spare slot but is NOT connected to the thunderbolt header. I do not have audio or webcam working on this 27” Thunderbolt Display - I have never been able to get this to work.
Display 3 iMac 2009 TDM - Vega 64 HDMI out going to a HDMI to DP converter going to a Late 2009 iMac running in target display mode. I have audio working on this display.
When first booting into Catalina after the upgrade I noticed that the LG 5k was back to 4k and my 27" Apple Thunderbolt Display stopped working once the OS finished booting.
Installing Whatevergreen brought back the 27" Apple Thunderbolt Display but left the LG 5K still at 4k
Adding agdpmod=pikera to my Clover boot arguments brought back the LG 5K to full 5k again - thanks sssteve!
I no longer appear to need the AppleGraphicsDevicePolicy patches (Black Screen Patch) so I have those disabled.
I have also removed my Intel UHD 630 config from Clover -> Devices -> Properties and replaced it with setting the Intel GFX parameter (0x3E928086) on that page and using the Inject Intel checkbox with ig-platform-id (0x3E920003) on the Clover -> Graphics Page.
All 3 displays working again perfectly, all being fed from the Vega 64 with full hardware acceleration (including Quick Look Previews) coming from the onboard Intel UHD 630.