My Mac will no longer read SD cards

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

This has been a common issue for many (including myself). I finally started to use an external reader then one day when I updated the OS it started to work again. Never could understand what took place.

That was so common on our iMacs we abandoned cards for portable SSDs for data transfer.

The only card usage is in my wife Nikon cameras and she either sends the photos wirelessly or uploads from the camera to the iMacs via a cable.

That's with two 2020 27" on Ventura and one 2015 21.5" on Monterey.

"they show up flawlessly in the camera's preview mode." Yep, same here. However, my comments still stand for us in relation to putting the Mac into the mix. Like I said, we limit cards to cameras only.

How old is the card? It is possible for them to go bad.

All SD cards or just that one?

Test a different SD card in the iMac.

Test that SD card in a external reader.

Test that SD care using a different Mac or PC.