OCLP explained to a complete moron please

CMP 5,1
RX580 non patched
Mac OS Mojave (want to run Sonoma)

Mac OS on SSD 1
Windows 10 on SSD 2

I flashed the CMP 5,1 FW to have a boot screen with the non flashed RX580 - works fine

I have OCLP installed
- booting into Windows works fine, everything works as expected
- booting into Mac OS DOES NOT WORK (I always get the crossed circle sign in the boot picker after selecting MacOS)
- booting into Mac OS Mojave after booting while holding the option key works and Mac OS Mojave works as expected

I have now tried troubleshooting this issue for months, having reinstalled OCLP more often than I installed Windows 95 during the 90's.
I moved physical drives and have removed all PCI Express slot drives (only 4 SATA drives remain and OSes are on physically different drives)
I tried installing OCLP on USB drives, the Mac OS system drives, on separate internal drives.
OSes and OCLP are updated to latest versions
trying to read through the troubleshooting gobbledygook for the gazillionth time only fries my brain
I just cannot seem to get OCLP to work

What I try to achieve:

- Run a dual boot CMP 5,1 (Windows + MacOS) and select which OS to boot on start
- upgrade Mac OS to Sonoma to be on the same built with my other Macs

It all looks and sounds so very easy on the many youtube videos but for a year of trying I just could not get this thing to run.