FYI it’s Full Time _Equivalent_. E.g a new feature may require 1 FTE for 1 week. That doesn’t necessarily mean 1 person 1 week. It may be 2 people 2.5 days each, etc.
Looks like https://www.arqbackup.com/ might meet your needs. I haven’t used it personally though, my only reservation has been that it’s not open source.
The unofficial google drive linux CLI supports encryption with custom keys too.
Interfaces were created because you can't extend multiple classes. It's the languages way of allowing polymorphism. You very well can create base classes and extend from them, but you will likely come across a time where you want to inherit from different things and interfaces are the only way to do that.