Just my photo library is over 1.5 TiB, there just not way to have long-term offline backups of that, you have no choice but to run and maintain a file server and cloud backups
I've had really bad luck with the reliability of mobile HDDs in storage. Try to use them after 3-4 years and they just die. Maybe I've just been unlucky?
You do realize that HDDs can randomly break, right? To keep a proper backup you need to have multiple copies of all your data and also regularly access / copy it.