I think boring, standard solutions are actually pretty effective. We use commercial cloud cold storage for primary backup, with a self-hosted cache layer in front to drive down cost. Commercial off-the-shelf storage is also the best characterized and has a proven track record.

NIST has some great reviews of the stability of optical media, and it’s quite good, done for the library of Congress.

DNA storage would enable some pretty crazy storage density, but ensuring there’s a compatible reader around in 30 years might be difficult