How to dual boot (Catalina - High Sierra) from a iMac 2020, 27", T2?
Your sticking point is actually much simpler than that.
No Mac can boot to an OS older than itself.
There's the occasional edge-case when a new Mac is released right on the cusp of a new OS, but otherwise, it's simply not possible. There are no drivers for the hardware in the older OS, as the hardware hadn't been invented yet, so Apple stops you from even trying.
Your only solution would be to run High Sierra in a Virtual Machine.