Even a single copy on M-DISC in a media-grade fire resistant safe is more likely to survive the next 35 years intact and readable than the alternatives. Cloud, hard disk arrays, tapes, whatever. The total maintenance cost over this time is dramatically lower than the alternatives, too (virtually zero).
And I would not worry about availability of readers in 35 years. I don't see passive physical media completely going away. Passive media carrier that does not include electronics. There will always be a niche for that. The drives you will buy in 30 years are likely accept the same 12cm disc form we have been using for the last 35 years and be backward compatible with CD, DVD, Blu-ray, Violet-ray, UV-ray, Gen1 holographic, Gen2 quantum, Gen4 quarkstore and Gen4 planckstore disks :-)