well, teak421, you'd probably have to find some more precise monitor calibration software/calibration tools, although basic ones are built into the Mac OS, and as the software and hardware have advanced, so has the level of calibration you can achieve. This link....https://blog.conradchavez.com/2015/10/27/new-os-x-color-profiles-strengthen-mac-digital-cinema-support/ says that Mac OS El Capitan does have a Rec-709 colour profiles, so I have to assume that your new iMac 2020 supports it. as you're running Catalina.... so no problem there. you'd probably have to use a calibration probe and the software that comes with it, assuming that there's a Mac version (Catalina version) of it....and there very may well be one...)
hope this helps you a bit
john b