Yes discipline is good as an investor, but just decipline alone won't stop you from tampering with you'd investment, especially in cases where difficult situation arise, St at times you add left with no choice than to tamper with your holdings, so as much as character is good, having a solid foundation for your investment is also good, like having a good emergency funds built up and also planning out various senerios out so you wont end up I panic from over expectations and sell your asset. And having multiple sources of income doesn't guarantee that you would DCA properly, cause if your expense are as high as your income you won't t still be able to allocate any part of your income to bitcoin since a large chunk is going to expense and you won't even be able to save up your emergency funds. So instead of getting multiple jobs it's good to find means to chunk down your expenses and know your numbers properly, this way even with one income source you are well prepared to Carry on with your investment.
Discipline plays a vital role in anything that we are doing, especially on achieving a goal to be a successful investor in a long time bitcoin journey. This is because you will be on the roller coaster for 4-6yrs and above. If you are not discipline to be consistent in using DCA approach as a beginner on your bitcoin journey, you will not be able to build up your bitcoin portfolio to a great height, because you will invest reluctantly.
What you said @Troytech is right but to only some certain investors which are the poor and average class that have more expenses to take care of, but discipline applies to all category of investors. Take example of a rich guy MrX, that has every resources to take advantage of having a significant amount of bitcoin in his portfolio, but because he is not discipline and feel he has everything becomes he is rich and chose to invest in bitcoin using DCA method but he hardly meet up with this because he lacks discipline. On the other hand MrY is not rich but have a means of income which after all calculations done, he can take care of his monthly expenses and emergency funds. He still could DCA regularly, and due to discipline, he was consistent with increasing his bitcoin portfolio for 15yrs. MrY will have more bitcoin than MrX, during the same duration of 15yrs. Sometimes, before you know it MrY bitcoin accumulation can bring him closer to the level of riches of MrX, because of compounding profit.
However, if a rich guy like MrX, let's say MrZ is disciplined in his bitcoin investment journey and invested on the same duration of 15yrs with MrY, he will have more bitcoin in his portfolio than MrY. This is because he will use his wealth to take advantage of investing aggressively in bitcoin using all three method of bitcoin accumulation strategy because other of his investment can serve as his emergency and reserve funds, so there is no cause for alarm.