thunderbolt cable

The display on my late-2012 build 27” iMac has died — computer still turns on but that’s all. Apple no longer supports/provides replacement parts and I’m not interested in trying to find 3rd-party repair.

I have a MacBook Pro (2015 build) and a new 24” iMac (M1, 2021). I would like to use either of these as an external display so I can erase the fusion drive of the 27” iMac. I am having trouble determining which Thunderbolt-to-Thunderbolt cable to use. I think the 2012 iMac and the 2015 MacBook use the same Thunderbolt port shape; I know the 2021 iMac's Thunderbolt port is not compatible.

Any suggestions about cable type and sources much appreciated.

Tim