Before investing in bitcoin, all of the guiding principles should be taken into account. As you mentioned, the first is financial stability. The second is knowledge, which will prevent you from being taken advantage of during the introduction process. The third is to take security seriously and know how to activate good security measures. Returning to financial stability, you stated that if you don't have other sources of income, you will use your investment as a repository for an emergency fund, which is bad for business. and since investing in bitcoin is a long time business patience is another factor to be considered.
All the principles, financial stability, security measures, and knowledge are good factors to consider;
What about the risk involved?
In my own understanding, I think risk taking is also an important factor to be considered before embarking on a bitcoin investment process because of the techniques behind it, and the market instability.
Risk taking is part of the knowledge that's needed to be acquired about Bitcoin investment so this person already included what you are trying to say, I will like to add more by saying, extra income source helps your investment more than not having an income, on the risk side you will feel comfortable buying Bitcoin and you will be able to take more risks if you have others ways to survive.
Job is very important or other ways that can bring in money for you and that's why I believe this advice from OP is precise, many people likes to get involved with Bitcoin and they are motivated to invest in Bitcoin, but with money they aren't ready to lose, and that's because they don't have source of income, so if Bitcoin doesn't pump in a year they get tired of holding very quick and the thought of maybe selling and move on with their lives will set in.
Source of income will help you takes your investment journey for years to come without the need to worry about the money you used to invest.