Using Migration assistant with one keyboard.

Option 1) Connect your old iMac's Time Machine backup drive to the new Mac mini and migrate from it.

see > Transfer to a new Mac with Migration Assistant

Option 2) Dig out, borrow or buy an inexpensive USB keyboard and mouse to get thru the initial Mac mini setup.

for example: > https://www.logitech.com/en-us/shop/p/mk120-usb-keyboard-mouse.920-002565?

Note: a USB-C to USB adapter is required to plug USB-A devices into the new Mac mini.

for some examples: > USB-C to USB Adapter - Apple

> https://www.amazon.com/dp/B07PDLQ555/ref=sspa_dk_detail_0?

> https://www.amazon.com/dp/B08Z768LCW/ref=sspa_dk_detail_1?

Dolphin31 wrote:

... This is impossible, whatever I do, the keyboard will always reconnect to the iMac and I cannot connect to the Mini to allow me to input the password. Befroe I throw the Mac Mini into the dustbin, does anyone have any ideas? Please?

Yes, get yourself another keyboard & mouse. All you need is an inexpensive USB keyboard. You do not need to invest in another wireless keyboard. And IMHO, for setup a wired kbd & mouse is preferable. If you don't have one yourself, a friend or neighbor may be willing to loan you one of theirs to do your setup & migration.

Do you have a Time Machine backup of your iMac? Using Time Machine with Migration Assistant is the preferred way to migrate user data.

If you don't have Time Machine backups to use try connecting the wireless keyboard with its charging cable so it acts as a wired keyboard.