Bits per [cm²|cm³|kg] is interesting like you get with cuneiform ceramic tablets[1], this one get about 1 word per cm² and cuneiform is crazy dense, I have no real grasp of how sumerian or akkadian words worked. I think it was heavily context based because from some lecture[2] at the British Museum.
I have seen people do ceramics where information was stacked in layers and had to be destroyed to extract. The ultimate form of shifting media to preserve and read information. I guess that could done with better resolution with 3D printed Zirconia (0.1 mm³ blobs) so 1Mb /cm³
Edit: this idea of a cold storage is from Footfall by Niven and Pournelle, where information was stored on monoliths where layers could be incrementally extracted with tools documented on the above layers. i.e. start with 0.1 bit per m² and go down, done with the hand wavy handling of practical problems in science fiction.
[1] https://www.bookandsword.com/2016/10/29/the-information-dens...
I don't recall the Fithp's artefacts requiring destruction to read; I thought they were just created with (presumably) lasers, writing the information in a way that would resist erosion - if one does get eroded, you just slice the eroded part away, re-revealing it again.
I do not remember, and tried not not imply destruction. It is just the easiest way to do it on your own with ceramics.
Akkadian is/was syllabic. The language is pretty well preserved I believe, some say there is more text in Akkadian than in classical Latin[^1].
[^1]: Can't find the source right now, so take this with a grain of salt.