By coincidence, I'd been looking into the problem of bitrot and what options exist. Hard drives are a starting point and last longer than removable discs. If going for optical, M-DISC seems to be by far the best option.
https://uncentered.saigonist.com/decentralize-your-data-part... https://uncentered.saigonist.com/decentralize-your-data-part...