I will explore the ability to work with historic data. What do you mean by “subcategories”?
As far as making it more “Savings Budget” like, in the same way you want to separate Categories from Budget, I think Budget Planning needs to be separated from evaluating the results of that budgeting. My first attempt at that is Budget Status, an alternative to Savings Budget. I’d love to get feedback on that template if you’d like to have a look. I agree it would be nice for Savings Budget to be able to update Budget Plan, but that would be something @randy would need to implement, which I’m guessing isn’t an easy thing to do.