Hi, Bondia,
There is no real answer, here. Basically, it is not realistic that any one media storage device will protect and provide access to your media for long term, like 20 years. Safest is to follow the rule of 3, and store in the cloud, optical disc, and physical drive. They each have their pros and cons. Right now the most universal format is Mp4, that you would get if you exported from iMovie with the High Quality setting. However, every 3 years or so you should refurbish and copy to whatever storage types and formats are available at the time. The digital world constantly mutates, with formats and storage devices becoming obsolete. Optical drives may not be available in 10 years. Hard drives deteriorate every 3-5 years. Optical discs can be scratched. All physical storage types can become damaged in a fire or flood. Physical storage media can be stolen or misplaced. Safest to store at least one copy in a secure place off-premises. A cloud storage provider possibly will no longer be around in 10 years. And if you encrypt, passwords can be lost. Storing in the cloud can also have issues with access by your heirs, so you need to carefully read the terms and conditions of the cloud storage provider and set your preferences accordingly.
Take a look at the excellent discussion in this link, and I think that all or most of your questions will be answered:
https://www.techspot.com/community/topics/whats-the-best-long-term-physical-storage-device.252008/
My take, and it's just mine because others may disagree, is that I would edit your vids with iMovie and export at the High Quality setting, that will give you Mp4 videos. Keep duplicates of your original unedited media. Mp4 is a high quality viewing format that is very popular and used at the moment. Store the media following the rule of 3 mentioned above. I think at the moment that a physical drive would provide the easiest access for your family, and the media could always be uploaded to the cloud from there or physically transferred to family members by thumb drive or disc, or burned to optical disc for distribution. You can't count on physical drives not to fail after 3-5 years. So always refurbish and recopy from time to time, and keep a master duplicate of your original unedited vids, as copying can cause some deterioration in itself.
-- Rich