Thank you for the suggestions, sadly I see I'm not alone in this situation.
I can't put this matter aside, despite warnings of it not being a healing process.

Situations I will take up are ignorance of the ethical code, wrongful termination, breaches of confidentiality, misrepresenting skills and qualifications, not supervising the work and a few more...

I appreciate the idea of the mediator.
The organization in question is the Polish Psychological Association,
Portal PTP - CODE OF PROFESSIONAL ETHICS FOR THE PSYCHOLOGIST

which I hope is serious, but I don't really know.

A mediator might be the best option to start with? Yet I'm told by another therapist abroad that, if the violations had occured abroad, she'd lose the licence for sure.

Yes, she would cooperate with the mediating process. In fact she herself is ok with me contacting the organization, we talked about it.

I have documentation to prove the truth of what I claim, for example recordings.
I want the truth, but justice also.

Would it be reasonable to try mediation first, then a lawyer? Would that prevent me from hiring a lawyer later, if unsuccessful? Any pitfalls I don't see? I just want peace, and hope I'm wrong about everything, and was treated fairly, but it seems really unlikely.

Thank you so much, sincerely. Losing a T is bad enough as it is..
Background education is psychologist, licensed.