old boss wants me back, should I dare or not?

I quit my job after three years, only the last year of which was awful. The reasons were that I was assigned tasks that didn't match my professional training, a lot of stress with management (not the boss, but those below him), several failed interviews, and, ultimately, the below-average pay (the equivalent of €21/hour). I found a job that I'm very happy with now: eight hours less per week, four days a week, and still a significantly higher salary (the equivalent of €30/hour).

Now my ex-boss (who is actually very easy to get along with) wrote to me and asked me how things are going in the new position and that I am very welcome to come back, also under similar conditions and again ONLY in the field of responsibility that corresponds to my training.

I currently have a part-time job (15€/h) that I do two days a week; that means I work a 6-day week.

I'm toying with the idea of ​​quitting my part-time job and going back to my old employer, working twice a month. That would give me the same pay as my current main job, with the same conditions, and I'd finally have weekends again, and even three days off in a row twice a month. It's not easy for me to make a decision right now because I'm very happy in my part-time job, my boss is super friendly, and they've been hiring for a long time. I don't think much of myself. I really don't want to work six days a week when it could be less… I've been doing it for a year now. Do you have any experience with going back to my old employer? What would you do? How can I tell my boss from my part-time job? I was always complaining about my old employer.