Issues with bootable USB

I’m not sure if this is the right place to ask this question, and I’d be happy to post in a better place if someone directs me.

My wife has a 2014 21.5-inch Intel iMac that is currently running on MacOS Big Sur, the latest available for that model. I have the computer booting from an external SSD and wiped the internal HDD because it was so slow.

I am currently trying to use a MacOS Sonoma bootable USB with OCLP to upgrade her iMac to Sonoma. I know the boot drive works because I used it to successfully complete this task with her 2015 MacBook Air. However, when I plug the USB drive into the computer, the external SSD boot drive is no longer recognized as a startup disk. So the iMac just gets an error message when I try to turn it on with the USB drive plugged in.

I tried resetting the NVRAM 3 times and still no luck. Does anybody have any other fixes to this problem with the boot drives, or another way to use OCLP to install Sonoma on the iMac without using a USB drive? Also, the fix must involve being able to install it without wiping the SSD, because she has an absurd amount of pictures on here that she has organized and does not want to move around. lol Thanks in advance!