Transmit is as "easy" as one could imagine software of that type being.
You do have to know what a file is and what a directory is, mind you, which is something I can non-ironically say does rule out half of GenZ or anyone else raised in the postmodern era, where 'content' just lives 'in' an 'app' and can be searched for (and if you're lucky, found). But I don't think people of that minimum level of sophistication are in the market for products like Backblaze or S3 - they're just out there paying for more iCloud storage (or new laptops) because Apple said they are out of space.
I would love for as user friendly way to just use Backblaze or some other S3 compatible provider as my drive.
Edit: I guess that's sort of exactly what Transmit does, but I want something that is simple enough that anyone can use it.