The problem is seemingly not the SD cards given that they work fine with your old iMac and camera. Then we can troubleshoot the SD card unreadable on Mac Mini issue from the Mac device and the connection.
As it is a new M1 Mac Mini, it is highly unlikely that the new device is the culprit when the SD card is not readable on your Mac. The SD card is fine as it is readable on the old iMac and camera. Thus, only the connection between the M1 Mac Mini and the SD card is suspicious.
Mac Mini doesn't provide any SD slot so you can only use a USB hub/adapter to connect an SD card to Mac Mini. Make sure all the involved parts are in good condition, including the USB cable and the USB hub. Most importantly, you should insert the SD card into the reader correctly, and make certain that the metal contacts are downward and pointing in the direction of the computer. If you insert the SD card in the wrong way, then the SD card is not working on Mac.
