On the other hand though, if you are disciplined and playing the long game but don't have that much capital to invest which in this case is low discretionary income, you will hit ceiling very fast.
You will realize that your capital is not enough and investing little amount of money bits by bits is waste of time.
For maximum effect you need both high discretionary income and the discipline. If you are a rich guy, even better. You can invest and forget. Most people do this successfully.
Considering investment is long term, I don't think that investing with little amount of money gradually with time is a waste of time. If it is the best as you can do, it's the best for your bitcoin investment and accumulation. Even so, if through your experience, you actually feel that Bitcoin is a good investment asset, you will likely have bigger motivation over time to work harder in order to get bigger discretionary income.
It needs time for you to gain experience, get profit from your investment so that you won't satisfy with it and learn more, work harder and smarter for higher income. It's never bad to start small, as it can help you avoid big loss while give you experience with time, and adjust yourself, your investment strategy and practice so you will have better result than another investor who spend same capital like your do, but with only one purchase.