Ruling according to the testimony?

I understand, so a few questions, on what basis does the rabbi say this? What are his sources?
Because from what I heard, the consensus is actually the opposite of what the rabbi says - that a person must accept Sephardic law if he was born Sephardic and vice versa, and there are no changes on the subject, no matter what his opinion is.

I also heard one rabbi say that a person is obligated to follow the laws of his community because of the verse “You shall not forsake the law of your mother” What would the rabbi say about this verse?

Indeed, the accepted approach is not like that, but that is my opinion. They also have no sources, and at least I have an opinion.
“Do not forsake” deals with customs. But when I have my own halakhic position, I must act accordingly. A custom is intended for situations in which I do not have a position. The Jerusalemite writes about this: If you do not know the most beautiful of women, go out after the flock and pasture your kids in the shepherds' lodgings”. If the halakhic law is sufficient for you (if you do not know), follow the custom (after the flock in the shepherds' lodgings). It makes no sense to oblige me to commit offenses (according to my opinion) just because my testimony believes that they are not offenses.
The accepted approach does not recognize the value of autonomy, according to which a person should act according to his own understanding.
See my article on authority and autonomy in jurisprudence.