Wireless Keyboard Not Pairing

I was gifted an iMac 27" with wireless mouse and keyboard (old style). The previous owner did not reformat the HD, etc. prior to giving it to me. When turning it on, the mouse and keyboard failed to pair via Bluetooth, even though both devices were listed. I installed 3 sets of new batteries in each (all from different packages). After multiple attempts to pair, I connected a wired keyboard and mouse. I then reformatted and installed the latest version of Monterey 12.7.2. I then attempted to pair the keyboard and mouse again. After several attempts, I was able to pair the mouse. I am still not able to pair the keyboard. Green light on bottom flashes and the computer finds it and lists it in the Bluetooth device list. I click "Connect" and the button greys for about 30 seconds, fails to pair, then then returns to "Connect". Keyboard is next to computer with no interference, fresh batteries, no other Bluetooth devices nearby. I should also note that both the mouse and keyboard show the names the previous owner assigned. I've searched several forums and YouTube for solutions, all of which have failed.