Firmware Password suddenly required for startup commands (can't access Recovery Mode or Boot Discs)

I updated my iMac to Big Sur (from Mojave) on Friday. Anticipating some potential issues, I made a time machine backup of all of my files beforehand and made a bootable USB disc with Mojave on it to make the process of reverting easy.

The upgrade appeared to go smoothly, but some of the compatibility issues I feared became a larger problem than expected. I decided to revert back to Mojave.

With my USB copy of Mojave and my recent Time Machine backup plugged in, I rebooted my iMac and held Command+R on my keyboard. This brought me to black screen with a lock asking for my Firmware password. This is strange because I have never ever set a Firmware Password nor have I heard of one since I've had this iMac (bought brand new in 2015). I tried every possible password I could think of, but could not get passed this screen. I can reboot into Big Sur, but that is it - all other startup keyboard commands bring me to a lock screen.

I Googled and found the terminal command to check the status of my Firmware Password. It shows a Firmware password is set and the mode is "command". Through the options given in Terminal under sudo firmwarepasswd, I can generate a 'firmware password recovery key' using -unlockseed, but searching online it appears only Apple can use this code (at a price I can't afford) to then have my physically bring the device in to unlock.

Can anyone help? I've tried everything and it's the strangest thing. I do not have any other personal Apple devices. This is posted from the same late 2015 27" iMac in question.

THANK YOU!