Plug your external working Catalina USB, after iMac power-on hold alt-option key for 20 seconds, then click the right arrow, and press-enter, if you see LED light activity from your external USB that means you're booting from there, so you can access your iMac in GUI Mode.
If still no bootscreen, again repeat, the process, and click twice the right arrow, and press-enter.
You need to manual set again from a Startup Disk your main APFS HighSierra boot volume, since after a full installation or cloning (with CCC that I don't use), the main OSX boot disk even after a PRAM reset, is still the latest installed or cloned one, not the surely working APFS HighSierra.
These tests are enough fast since you have an SSD.
Probably something went wrong during your data migration, that migrated some Mojave caches incompatible with Catalina, due to the fact that apple splitted the System from the Data.