iMac 13,2 (late 2012) trying to get working again

I ruined my late 2012 iMac OpenCore install by upgrading to Sequoia + latest OCLP. am trying to go back to Sonoma 14.7 and OC 2.2 since that worked. I am getting stuck when I go to boot from USB by holding Option key - I end up at the Prohibited logo. Not sure what's wrong but I know the hardware is okay. I ended up doing a recovery and managed to get the ancient 10.8.5 reinstalled through an Internet-based recovery install.

My question is - what's causing the Prohibited boot error? How can I see any actual errors going on? I bought a new USB stick as I thought maybe that was corrupt - no change. I am making the media using the createinstallmedia command, and then doing Build and Install OpenCore with the Model set to iMac13,2. Not sure what else could be wrong. I did wipe the disk fully prior to the Prohibited sign appearing, so not sure if I did something wrong in that process. But the fact that I somehow got back to 10.8.5 makes me thing I am missing something obvious. Anyone got any advice?

Thanks for your help!