In simple words, if someone wants to invest for a long term or buy in dip and hold, then the total income must be divided into 3 parts. 50% daily expenses + bills and 20% Emergency & Reserve fund, that is, first complete the emergency, then create a reserve and the remaining 30% can be invested and this is the safest long-term investment plan. Now if you want, set up a DCA strategy with your profile for dip buying or long-term holding. If you keep it like this, it will be clear which money is for survival, which is as a buffer and which is for real investment. There are also many other strategies for investment, but this is the one I like the most and I consider it the safest.
You are not in the position to determine how much is your discretionary income but your basic needs and weekly expenses will determine it. If you calculate how much is your basic need and monthly expenses it might be problematic for you to know the right amount of money that is your discretionary income and take not that your discretionary income is what you use to invest in bitcoin and build your emergency funds and other backup funds.
It's not the amount that we love to invest with that we must use to invest in order to avoid using beyond your discretionary income to invest in bitcoin because it will be a waste of time and gambling if after investing with the amount you feel is cool and you end up selling part of your bitcoin because you couldn't meet up with your basic needs and monthly expenses. This is why proper planning and right discretionary income should be figured out before using the money to buy bitcoin.
It is very important to have a long-term perspective to invest in Bitcoin, but there are many obstacles that you have to overcome in the long run. I agree with you. If you invest directly without saving for your essential expenses such as food, medical expenses, etc., you may encounter a different reality. Because when you try to avoid these, you may be forced to sell your long-term Bitcoins if you do not have the money when you need it the most. Not only will you be financially damaged, but you may also lose the ability to invest again in the future.