Unfortunately, currently produced mdisks are not the same as older ones. Requirements for media has been relaxed a while ago, most disks are not meant to last 1000 years anymore. Still, I don't know any viable alternative, besides maybe LTOs if you're lucky to know somebody with reasonable drive (not sure about US, in EU drives are quite expensive, and I mean used drives of course).

Edit: found Reddit discussion I learned that from: https://www.reddit.com/r/DataHoarder/comments/13693k0/new_de...