Instead of FAT32, use the flash drive mapping. Flash drive contain mapping from sectors to blocks. This is used to deal with flash erasure, wear leveling, and failures. Custom firmware could map the real filesystem to bad blocks.

Does the forensic software read the whole physical flash drive or just the logical one? They might if they want to get deleted files. Can they detect that there are lots of failures?

You need the wear levelling system for actual wear leveling, otherwise the drive won't last very long. Competent forensic analysis can check the firmware and the physical flash contents, although most forensic analysis is not especially competent.