On the first part, I agree, NASA's budget is basically constant in real terms over the past 2 decades or so. The bigger problem imo is not the total amount of money, but all the strings it's come attached with, and in particular constantly-changing strings. If NASA got $17 billion and the freedom to decide how it should be best used to advance scientific goals, that would be one thing, but instead every President and Congress has their own pet ideas about what NASA should do with the money, and so it ends up getting allocated to a rotating parade of prestige projects that end up going nowhere.
On the latter point, I don't see NASA as really less efficient when they're given freedom over designs. They have some pretty impressive low-cost-satellite programs that teams of scientists have put together out of small bits of discretionary funds. And while StartX is impressive for a private company, I think it'll be some time before they're doing NASA-level work. For example, none of the private sector companies seem to be even attempting scientifically useful things like space telescopes, Mars rovers, or interplanetary probes, even though there is no technological barrier to doing those (they're doable with literally decades-old technology).
On the latter point, I don't see NASA as really less efficient when they're given freedom over designs. They have some pretty impressive low-cost-satellite programs that teams of scientists have put together out of small bits of discretionary funds. And while StartX is impressive for a private company, I think it'll be some time before they're doing NASA-level work. For example, none of the private sector companies seem to be even attempting scientifically useful things like space telescopes, Mars rovers, or interplanetary probes, even though there is no technological barrier to doing those (they're doable with literally decades-old technology).