Might be a dumb idea but hear me out. How about sealing a reputable enterprise or consumer SSD in one of those anti static bags with a desiccant and then sealing that inside a pvc pipe also with desiccant and then burying it below the frost line? You'll just have to dig it up and refresh everything every couple of years, think 3 years at most iirc for consumer ones. Obviously this isn't a replacement for a backup solution just archival so no interaction with it. It'll protect it from the elements, house fires, flooding, temperature fluctuations pretty much everything and its cost effective. Hell you can even surround the hard drive bag in foam then stuff in the pvc pipe for added shock absorption. Make a map afterwards like a damn pirate (its night time so my bad if I sound deranged)

edit I took a nap: in hindsight I should've clarified. I went with an ssd in this idea since its more durable than a mechanical, better price for storage capacity compared to m-disc, and most likely to be compatible with other computers in the future in case you need it for whatever reason. Of course you can use another storage media, like m disc, just know of the drawbacks. Like needing a m-disc burner (~100$), several discs depending on how big of a capacity you need (price varies), pray that there's still a drive that can read m-disc in the future and know that's its gonna be slow when getting your data back regardless. All you would have to do to modify the idea would be getting a disc case that kinda suspends the disc so nothing is touching it's surfaces. Then the same idea: antistatic bag with desiccant, foam or even bubble wrap around it, stuffed in a pipe with desiccant buried below your frost line. People usually skip the "in optimal conditions" part when talking about m-disc but this way we get close to those optimal conditions