my first job we had a vault of ibm reel to reel tapes of old business data. our attitude if we were ever asked to pull any of the data was that we would probably only get one chance at it as the ferric material on on the tape had a disturbing tendency to flake off. note that there are techniques to reduce this, but we did not have the means or motivation to apply them.
And a pedantic observation on your correct point about optical disks. you can't make backups on pressed disks, only recordable ones.
My optical disk drive was $17 and has M disc writing compatibility, and my understanding is they are meant to be read by any CD reader.
You need to let flash cells rest before writing again if you want to achieve long retention periods, see section 4 in [1]. The same document says 100 years is expected if you only cycle it once a day, 10k times over 20 years (Fig 8).
[1]: https://www.infineon.com/dgdl/Infineon-AN217979_Endurance_an...
And magnetic drives will seize up. and optical disks get oxidized, and tapes stick together. long term archiving is a tricky endeavor.
It is however an interesting challenge. I think I would get acquainted with the low level protocol used by sd cards. then modify the sdmmc driver to get me an image of the card(errors and all). that is, without all the scsi disk layers doing their best to give you normalized access to the device. Or more realistically, hope someone more talented than me does the above.