When erasing media in Disk Utility, how can I change the default format?
There isn't a setting to change, so you might instead script the diskutil command to do the erase using AppleScript or another scripting language.

diskutil eraseVolume disk12345


The eraseVolume command just wipes the volume and recreates an empty filesystem to match what was there. You could build a little window to ask for the volume or perhaps even list all the disks and then tell you to insert the drive to be wiped.

You also might use system_profiler SPCardReaderDataType or the similar SPStorageDataType or SPUSBDataType depending on how the cards you wish to erase are connected to the Mac.