I don’t think there are any items in the store that grant scraps / dusts and they’re not transferrable between players so I don’t see how this would happen.
No, you’re not missing anything.
Getting all eight legendaries to Rank 30 is quite literally many months of grinding. Doing open-world “circles”, with the Treasure Hunter paragon tree active, is the most effective way to accomplish this but it is, as you say, absolutely mind-numbing.
Introducing some sort of catch-up mechanics for gear ranks was one of the things I suggested months ago…