Did you run Software Updates and made all the updates? Did you run SilentKnight to be sure that you got the most recent XProtect and MRT?
I checked for updates but didn't run SilentKnight (didn't know what it was...) I've booted back into my patched Catalina build as a have some work to do, but I ran SilentKnight to see what it does. The standout was that while I have SIP disabled, the text panel says:
❌ System Integrity Protection status: unknown (Custom Configuration).
Configuration:
Apple Internal: disabled
Kext Signing: disabled
Filesystem Protections: disabled
Debugging Restrictions: disabled
DTrace Restrictions: disabled
NVRAM Protections: disabled
BaseSystem Verification: disabled
This is an unsupported configuration, likely to break in the future and leave your machine in an unknown state.
Which probably isn't helping. I'll switch back to the Mojave install tomorrow and fix whatever issues SilentKnight throw up.
Thanks for the help so far, btw, it's appreciated.