AFAIK GPON uses encryption, so you actually get the traffic intended for all your neighbors but can't do anything with it. If you bring your own converter, you wouldn't be able to handle your own traffic either.

Usually yes, but it depends: https://pierrekim.github.io/blog/2016-11-01-gpon-ftth-networ...

Also the authentication might rely on weak secrets. I know my ISP provided FTTH router has a six letter password and a guessable username (derived from my last name), and I can't change either.

Though the research is quite old now. Couldn't find anything recent specifically for DT.