Job security has to be considered too when someone has finally decide to start his bitcoin investment journey. One has to be certain that your job has to be really secured, and not a kind of job that won't last long. Job security should not be underestimated because if you have chosen to invest through DCA make sure that your job or source of income isn't something that's going to be terminated on a short run.
I understand your emphasis on the stability of the source of funds but about job security, sometimes it will be somehow difficult to determine the security of the job. That being said, I do not see the need to use the though of job security as a factor because it could be like complicating the entire planning process. In addition, if you bring in so many unnecessary factors into the equation, it can only lead to discouragement, procrastination, and eventually lack of investment into Bitcoin.
I have known that there is no perfect thing in any business with Bitcoin investment inclusive. There is no perfect time, no perfect entry price, no perfect buying strategy, all we are doing is just take the best approach in the midst of many options. So we will not wait until we get the perfect job, perfect cashflow, and perfect everything before we start accumulating Bitcoin. When we are confronted with a situation of things like job security, the best thing to do is to assume the job is sustainable and will not be terminated any moment. We could play that way and hope for the best. If it is also a contract signed for certain number of years with possible chances of extension, we can plan with the earnings for the number of years of the contract without extension so that even if the extension did not happen, we will be fine. In this case, we will become more aggressive in seeking alternative sources of income within the time you have to the expiration of the contract.