Incase anyone ever needs the answer…I have a 27” 2010 iMac working in Target Display Mode with a M1 MacBook Pro using a miniDP to USB C adaptor that's capable of 4k60Hz.
iMac 21.5 Inch 3.06GHz Intel Core i3 Mid-2010 MC508LL/A
Apple iMac 27" Mid 2010 Core i5 2.8GHz 16GB RAM 1TB HDD Radeon HD 5750 CHIPPED | eBay