You only need a little income to invest. Even a small amount of income can be seen as an investment if you have the mindset to invest.
As far as bitcoin investment is concerned, you need a source of income that will allow you to have discretionary income after you have sorted out your daily expenses so that you will have the money to invest in bitcoin in such a way that it will not stop you from sorting out your daily expenses, which might get you off the game partially or permanently.
You are right, we need to have a source of income that allows us to have discretionary income however if we have a job that don't allow us to have a discretionary income we need to backup with a side hustle, some people who are working and still don't have a discretionary income is not because there salary is not good enough is because they have a bigger responsibility more than a normal human should have, they carry unnecessary responsibilities for example always sending money to there friends weekly or monthly as soon as there salary is being paid, there are some responsibility we should cut off.