Reviving a "dead" iMac?"

I purchased what I've been told is a 2020 iMac (27") that apparently got bricked. The seller told me that the power went out when he was performing an update. Since that time, it has not been bootable. Simple restarts result in Black screen with Apple Logo and fans on, but then cycles through restarts with black screens and fans. I tried restarting while holding various combinations: shift, command+r, shift+command+r, T (for target disk), control+command+P+R (PRAM reset), etc. None of these have "solved" the problem, but some of the restart options seem to do more than others.

The PRAM reset option, for example get me the Apple logo and the chime, and I've followed instructions to keep holding through to chimes, but then this goes back into the black screen cycle like a simple restart.

The command restart gets me to a point where I can select wireless network, log on to that, and start an internet recovery (with the spinning globe and status bar). This progresses through the status bar, but then restarts back to the network select screen.

Starting while holding Option allows me to select network, log on, then takes me to the hard disk icon (Macintosh HD) with the select arrow. Clicking the select arrow goes back into the black screen cycle that leads nowhere.

Target disk restart gets me to a black screen with the Thunderbolt and USB icons, but when connected to my MacBookPro with either USB-C to USB-C or USB to USB-C, I do not see the iMac in Finder.

I even downloaded and made a bootable USB (Sonoma) and tried to boot from that after restarting while holding Command. I could see and select the bootable USB, but after selecting it and clicking the arrow, it went back to the black screen cycle.

Any other suggestions?

Thanks in advance!!

I did not spend much on this (~$200), but it was a gamble I thought was reasonable.