NP here. If I may piggyback… can you tell me more about what you said about flash drives’ stability?
They are basically made up of capacitors, which hold a charge and will lose that charge if left without power for months/years. The same applies to SSDs in general. USB flash drives also tend to use rather low-quality flash storage, so data integrity is also an issue with them.
No digital storage is really shelf-stable. Magnetic tape (LTO, etc.) is probably the best, but even that needs to be stored in a standardized format (e.g. GNU tar) and a drive needs to be available for that format (LTO8, etc.). The best way to store this stuff is to have multiple copies in different physical locations both online and offline (mechanical hard disks are more-or-less shelf stable for several years, anyway). Compare the files periodically (e.g. by making md5 checksums) and replace any files which have become corrupted with another copy. Then, migrate the data to more modern storage devices over time.