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.
How to Connect Two 27" iMacs for a Dual Display Setup! - YouTube
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How To Use an iMac as a Monitor for a PC
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How To Use an iMac as a Monitor for a PC
How to Connect iMac to Monitor | use external monitor as a second screen for iMac Pro - YouTube