Should I join my previous company?

When does an employer have to join the employers' association?

The following situation: I work in a service company with 7 permanent employees (100%), 3 permanent employees (70%) and 6 temporary workers. Now the change to the minimum wage is pending for us, affects me only indirectly, because I have a firm hold on. But for years there has been a lot of trouble with our boss, because she believes "she" is the collective agreement (according to the boss, there is no valid collective agreement for our TAXI NRW branch), degenerates into shouting and untenable accusations. I'm now in the 10th year, haven't had one since then. Christmas, Easter and New Years Eve had more free time. Our company management has not yet given any explanation as to how this will continue from January. Now I have heard from a colleague that we are planning to cut the break times and idle times together in order to deduct those from the hours, furthermore we should receive new contracts, which I do not want. Last month 2 of my colleagues joined Verdi, but I don't know whether I should join too. I have private legal protection insurance, which includes professional legal protection. I'm torn as to what to do. Now I would like to receive opinions on this issue, for or against union, for or against legal protection. I would be happy to receive many, technically good answers.