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) Tested and Powers On | eBay
Apple iMac 27-inch, Mid 2010 Memory 4 GB. Not Used Since 2015 | eBay
OWC Turnkey Upgrade Program For 2010 Apple iMac 27" Now Offers Ability To Add Up To Three Solid State Drives Or Additional Combinations Of HD, SSD, & eSATA
IMAC 27 inch - mid 2010 pentru piese Bucuresti Sectorul 5 • OLX.ro