Many supervisors exert control by being your only source of financial stability. Find alternate ways to get paid.

Don’t TA for them if you have a choice, get a secret job outside the university, whatever it takes to not be dependent.

Depending on your institution, you may have different funding opportunities available to you. Many grad students are funded by a combination of TA work, RA work, and grants. Some institutions consider the TA/RA work to be a “real” job, some may consider TA/RA work to be a job for accounting purposes but not in practice, some may consider TA/RA work to be a necessary part of the grad school contract but not governed by law. Grants are almost always directed through the institution to some degree.

You need a source of money that is not from inside the institution in any way. Some institutions prohibit this or make it difficult for grads to work elsewhere. Some institutions encourage it. The key for you is to not be dependent on your supervisor or institution for funding.

This may be contrary to the logic you have been living off of so far. For example, institutions often make it seem that outside work “takes you away” from your degree or will increase the chances that you will not graduate. That is not your problem, that is their problem. If you decide something is more important than you’re degree, then you’re right. There’s nothing wrong with leaving your degree for something you enjoy or value more.

However, realistically, this is just a scare tactic. The reality is that you are being taught by a group of people who did not leave the institution: they expect the same from you. Likely, they have no concept of what the “outside world” can provide. They are likely just prejudiced against things that they do not understand because it helps them to control you.

Choose to diversify your interests. This will never hurt you in the long run. If you TA for a different professor than your supervisor, you have gained a professional contact. Then, if your supervisor decides to not give you a TA job, you can ask the other professor. Even if it’s not in your area, it may turn out to be beneficial.
Similarly, choose to RA for another professor if you can. Again, this will expand your professional network. You have the time and the space to do this, even if your supervisor believes you do not. If you are in such an abusive workplace that your supervisor will not allow you to do so, get a secret job elsewhere. If you do not even have time for this, take leave or go to therapy.

For outside work: remember that the institution does not have jurisdiction outside of itself. It cannot stop you from acting in your own best interest by finding a job. It can only punish you to the extent that the internal policies are enforceable. If they don’t know about your job, they can’t punish you. Don’t give them the chance.