For my own use, I researched and successfully tested methods of integrating an M1 Mac mini with a Thunderbolt Display and up to to two iMacs in Target Display Mode. My focus was on minimizing expense since both my Late 2009 iMac and Thunderbolt Display are aging. So I've identified low cost interface items that are currently available on Amazon. For a single iMac, the interface items are less than $40. For the Thunderbolt Display, the cost is < $50. The attached PDF should make clear how to get this done. You can go all out with three displays or just one. Note that you can use only one of the M1 Mini's Thunderbolt ports for a display unless you turn to something like a DisplayLink adapter. The options I've included are focused on employing existing Apple screens that you may already own and they should not add additional calculation burden on the CPU in the way DisplayLink appears to. Some of this should be applicable to M1 laptops and I hope this helps someone!
