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 27-inch Mid-2010 Not Displaying : r/mac
Imac 27 i7 mid-2010 €350 №4531814 in Limassol - Desktops, servers - sell, buy, ads on bazaraki.com