Is it forbidden to work as an IT systems consultant for an entity whose duties may include overseeing an electronic system that promotes musical concerts, taking into account that he has accepted the offer and resigned from his previous job, and that it offers financial and psychological benefits and time with family?

It does not necessitate ruling your new job as unlawful, due to the possibility of overseeing an already existing website that may contain promotion for musical concerts, and there is another possibility in your previous job that required working on similar projects. Therefore, it is not unlawful for you to start your new job, provided you strive to avoid specific objectionable matters therein.