Unable to wipe and reset 2019 27" iMac for resale

I've got a base model 2019 27" iMac, 3Ghz 6-core intel i5, with 1TB Fusion Drive, running macOS Sonoma 14.1.2. I'm in the process of wiping the iMac to sell it, but discovered that since this model does not have the T2 chip, I can't use the Erase all Contents and Settings option under Transfer & Reset in System Setting, but instead I have to do it via keyboard shortcut when restarting (Command+Option+R).

The issue I've hit is that my iMac is ignoring all keyboard inputs when restarting from the apple bluetooth keyboard. I've tried the option to wipe the hard drive and reinstall macOS, held down the Shift key for safe boot mode and tried clearing the PRAM, the iMac just restarts and ignores all of these attempts.

I've then gone into the Terminal Window to reboot the PRAM there with sudo nvram -c, but I get the following error message:

nvram: Error clearing firmware variable fmm-mobileme-token-FMM: (iokit/common) not permitted

nvram: Error clearing firmware variable fmm-mobileme-token-FMM-BridgeHasAccount: (iokit/common) not permitted

nvram: Error clearing firmware variable fmm-computer-name: (iokit/common) not permitted

I understand this is due to password protection on the Firmware, which according to the website and can only remove by booting into recovery Mode to turn it off, which I can't do as my iMac is ignoring all keyboard commands on startup.

Any suggestions? I've backed up all data and have all my file on my MacBook Air which has replaced this iMac.

Thanks