The most helpful parts of Stoicism for me are understanding and accepting that awful things might happen I can't do anything about, the hedonic treadmill exists and a little more money, a car, a speedboat, or whatever I currently desire probably won't make me happy in the long term.
If I spend a week or a month in tatters over something minor, I can learn to adjust myself rather than trying to change the world to fix my own head. Long-term happiness is a mixture of changing the world to fit my goals and changing my goals and what I demand from it to fit the world - to learn that I cannot get everything I want.
Where I disagree with Stoicism is the ideas that my goal should be to optimize myself to some sort of perfection, and that I can ever have perfect control over myself mentally. My thoughts themselves live inside a biological brain which is affected by its construction, genetics, hormones, and many other physical influences. Beyond this, the idea that any specific thing must be the moral goal of all persons seems, as it is put here, like a gross presumption. But I never went in that far to the philosophy.
What does strike me, though, is your identification of various "self-improvement" techniques together under the Stoic banner - I've recently been confronting aspects of my personality in that sense and it made me jump a bit to read your description - it makes me think there is a bit of "There" there I should reconsider.
If by "profligate" he means "wasteful in the use of resources" then this whole thesis falls apart immediately.
It's actually Nietzsche -- "Beyond Good and Evil," Part 1, §9. I have very serious reservations about Nietzsche, but he doesn't pull his punches.