are the codes constant? and when someone types something you checked against the list of existing code to see if there is a match? or can code be declared dynamically?
How many codes do you think you'll have to accommodate? You could store a few more in RAM on your Uno, depending on what else is using it. You could store a lot more in progmem before you need to deal with an SD card. You could upgrade to hardware with more RAM: Mega, Teensy, Feather M4 etc.Of course, the nice thing about the SD card is that you don't have to tweak the code to add new entries.