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.
Apple - iMac - (27 inch, Mid 2010) - i5, 8GB, 256GB-SSD, High Sierra – Ordinateur St Joseph
OWC Internal SSD DIY Kit for Apple 27-inch iMac 2010
Apple iMac 27-inch, Mid 2010 Memory 4 GB. Not Used Since 2015 | eBay