For the life of me I cannot boot from external NVMe disk

I've installed Sonoma twice and once again over the last installation but it's always the same. I get either a black screen, a logo or if I'm lucky it gets about halfway and hangs whether booting from USB or no. The internal SSD is running the last supported OS Monterey on a 2015 iMac 17,1. I've zapped the pram and don't know what else to do