It is very good for someone that is into bitcoin investment to use only his discretionary income to invest in Bitcoin just like you said, it's saves you from emotional stress because if you invest with money you are supposed to use to pay bills when time comes for you to settle those bills and you don't have the money to do that you will start thinking and even dipping hands into your bitcoin investment will make you also think and from there you will become very stressed out emotionally it is better to prevent it by using only your Discretionary income to invest in Bitcoin.
Some people that were investing in Bitcoin that later stop investing was because of emotional stress that comes with bitcoin investment and they had this emotional stress because they don't use only their discretionary income they use money that Is meant for important things to invest and later they start complaining.
Bitcoin investment must be our top priority but we must not over stressed our self in doing that investment. Best way to go for Bitcoin investment is to generate some kind of discretionary income. The salary we receive on monthly basis must be used for our daily requirements and some portion of that must be reserved for emergency funds. Those who do job can do online work to generate discretionary income like participating in signature campaign or work on freelance platforms. Their is no excuse these days for not generating discretionary income.
People keep stressing themselves so much about what portion of their income they should invest in bitcoin. Bitcoin investment is open for all, even the low income earners. It is good that bitcoin is highly divisible into smaller and affordable portions, if you don't have large capital to invest, then you simply DCA at the rate which is convenient and very comfortable for you. If after investing that small portion you still feel that amount invested is not something you can forgo , then I will assume that such a gambler is not a risk taker and is not fit to invest in bitcoin.
Investing only discretionary income does not take away the risk of losing that money. If you lose your discretionary income, you would get pained because it is still your money.