This is a worthy and very difficult problem. I don’t think there is a perfect answer but I’ll give my viewpoint.
Lets assume that you have a modest number of terabytes of personal data. 100,000 photos, 1000 movies, pretty much unlimited text files, like that.
You need a threat model. The obvious threats are equipment failure, theft, fire, natural disasters, misplaced trust in friends and family, criminal activity, and government activity. You also need to account for user error (you!)
To deal with equipment (drives, computers) failures, you need multiple backups that have independent failure mechanisms. External drives (not plugged in at the same time!) raid storage, cloud storage, things like that.
To deal with theft and fire, you need backups that are not in your house or apartment. Cloud storage might work, keeping backups at your cabin, or a (far away) friend’s house.
To deal with natural disasters, you need backups that are not in a single geographical area.
To deal with misplaced trust, you need to leave backups with at least two different trust boundaries. A friend, a lawyer, a safe deposit box.
To deal with criminals, you need to make sure your data cannot be ransomed. The easy way to do this is offline backups.
To deal with inimical governments, well if you have problems like that you might benefit from professional assistance!
What I do is to use Apple Time Machine to keep two different backup datasets. One is on an external drive and one is on a file server. The copy on the external drive could be encrypted by ransomware, but I don’t think the NAS is vulnerable to simple versions of ransomware. I then pay for a cloud backup service that works independently of Time Machine. I also occasionally copy stuff to external hard drives that I keep on the shelf.
Equipment failures are dealt with by having multiple backups that each keep a history (TimeMachine like). Fires, theft, and disasters are dealt with (I hope!) by having cloud backups. Make sure you keep the passwords and encryption keys somewhere safe!
I am probably not well protected against misplaced trust. I hope I don’t have that sort of problem in my life.
As far as criminals are concerned, I am probably not well protected against a targeted attack by a skilled adversary, but I’m not worried about botnets or script kiddies. They have far easier targets than me! The one time that I know about that I had a server compromised, I erased the whole thing and restored files from an old backup.
I am probably not well protected against a government, but I don’t have problems that I know about. If you do you need to read up on operational security and shouldn’t be keeping online data anyway.
As for things like Bluray, I wouldn’t trust them for archival storage. There are some <very expensive> metal film recorders that might be good for 100 years, but they are not for the average joe. For choosing good hard drives, see the BackBlaze and Google drive reliability reports that come out every few months. It is a moving target.
Doing a reasonable job on backups is the main point. Two external drives and a cloud backup service are about the minimum I would recommend. Plan to pay $100/year for the cloud service and to replace one drive per year. Do keep the old ones but don’t count on any particular drive being usable after even a couple of years on the shelf.