As far as I know flash memory should last at least ten years. After that period of time I would either copy the data, erase and write again or look for other tech. So this requires manual intervention.
Which other? Magnetic tapes are often used for this purpose. Microfilms shall be much better. Paper is really resilient.
I would avoid any cloud, Google has a bad track record. The online access and requirements make it unreliable. In best case the don’t try to search the data or delete it by accident. As they did already.
As far as I know CDs last only ten years. DVDs maybe 30 years. I’ve it is a -ROM it last likely 100 years. But therefore you need special equipment. The M-Discs you mention are new for me but this sounds good. Are regular drives guaranteed to read it?
The most problematic thing isn’t just keeping the data. We also need to be able to read it again. Maybe we need to use good old stones ^^