Ok, so the “council of Jamnia” is a protestant argument that says basically this: because the Hebrews used only Hebrew scriptures written in Hebrew that means the Greek texts should be thrown out. Now bear in mind the Catholic Bible wasn’t even opposed until the 16th century. The "council of Jamnia wasn’t even proposed until the last 100-120 years. The reasoning behind the council of Jamnia is to have an excuse for not having 73 books in the bible. That all being said, Martin Luther didn’t even use this argument he just straight up got rid of them because they opposed his theology (Faith Alone). I think I remember him citing some ancient saints around the council of Rome when the canon was being debated that thought the dueterocanonicals shouldn’t be included but later submitted to the Pope’s decision to include them (I don’t remember the specific saints name but he often gets brought up when these questions are asked).
The reason why the Catholic Church uses the Septuagint argument is because like what was said previously, the New Testament quotes from it very often but they are using this argument simply to say the deuterocanon is inspired. all that being said, you are correct, there are books in the Septuagint that are not in the Catholic bible. HOWEVER you have to ask yourself the real question which is this: How do we even know what is inspired scripture and what is not? Well, the early church decided on this at the council of Rome in 382. They decided as a congregation of Bishops with the Pope what books should be in and what should be left out. It comes down to who has the AUTHORITY to make theological decisions. The Pope has the final say, and since in matters of Faith and Morals when making a dogmatic statement, he is infallibly led by the Holy Spirit these are the True Scriptures.
Without someone who we can trust to be led by the Holy Spirit we would all be running around making up our own beliefs and saying that “your truth is your truth, and my truth is my truth,” sound familiar? It’s because when everything is subjective Truth doesn’t exist. So when Martin Luther or the Eastern Orthodox decide they don’t need a Pope, they don’t need apostolic lineage to ensure the Holy Spirit is guiding the Faith, everything falls apart. things get added, taken away, churches break up, new denominations form, and separation between people ensues. You have to be Obedient to Christ’s words: “You are Peter, and on this rock I will build My Church” and stick with it, even if there is corruption, you find a way to fix it from the inside. you don’t just leave and try to figure it out yourself.