Decision about getting a job while completing MSc Studies
It's clear that you are reluctant to take on full-time employment after your recent experiences, and understandably so. However, this does not mean that you can't take on part-time work to help pay the bills. This is a fairly common phenomenon for master's students, particularly in countries where the master's degree is considered an "undergraduate" degree (such as in Europe). Exactly how much is up to you—you probably know how much time you could spend on external work without compromising your desired work-life balance.

If your bachelor's advisor is understanding and supportive of you, then he would likely be willing to let you work part-time on his project while you finish your studies. If he isn't willing to do so, then that might be a sign that he's not someone you should have as a long-term advisor. (A good research advisor and mentor will want what is best for both of you, not just what is best for the advisor!)