I guess I may need to buy new SD cards. The problem is that I don't understand exactly what's wrong with the cards I have... so I'm not sure what new cards to buy.
I would be quite surprised if this was an SD card issue, unless maybe if they're counterfeit. What you've encountered is unusual. It really *shouldn't* be this difficult.
People have run into problems with certain card readers.
Are 64GB cards generally not a good idea for use with CHDK? My understanding is that the FAT32 filesystem is only really meant for cards up to 32GB in size. Does that mean I'd be better off buying 32GB cards?
I've had no problems using 64 GB FAT32 cards in modern cameras (G7 X, SX710, SX730) with CHDK.
If you don't need 64 GB at a time, 32 GB are slightly more convenient, since you can format them anywhere. The price/GB isn't quite as good, but they're still pretty cheap.
And does it make a difference if the cards are SD, SDHC, or SDXC? I don't really know much about the distinctions between those card types.
SDHC only goes up to 32 GB, SDXC goes larger. Either should work. The original SD only goes up to 4 GB and is long obsolete. Wikipedia is good
https://en.wikipedia.org/wiki/SD_cardCards with U3 or V3 speed rating provide the fastest file writing on these cameras. Note the U ratings are confusingly not the same thing as the UHS-... standards. This cameras only implement UHS-I, so higher UHS rating cards will work, but not provide significant benefit.