A spare 32 GB card will work fine for the test. The tedious part is the long wait for the H2testw to complete its write + verify when hardware checking the empty card. Don't skip doing the H2testw part.Unfortunately no, all my remaining cards are 32GB, but when i boot the 3ds with the luma installer it shows Booted from ( SD / CTRNAND ) via B9S in the bottom left corner of the touchscreen, but it just continue to display the "i can't read the SD card bruh" message.
With your main SD card inserted in the 3DS, which one exactly does it say for Luma configuration: SD or CTRNAND?