I have virtually the same setup as yours and having issues doing a clean install of Catalina. Any chance you could share what your setup is? Clover version, kexts used, config.plist? Thanks in advance!
Also had same issue. Changed the system definition from iMac 14.2 to Macpro 5.1 and that worked for me. Did not enable IGPU in BIOS. Will see how it works longer term. Am now able to also new RAW (eg. NIKON NEF) also in preview and PDFs seems to be render a little faster.
Updated thru app store ok - did not lose sound (ALC1150) or USB3 functionality. Original installation was clean install via clover (3270) and patched for USB3.
Gigabyte GA-Z97X-UD5H; i7-4790K; PNY GTX 760