You should still test anyway. Many Nigerians are pleasantly surprised when they get their results back. Many have said, just like you, that they know their history going as far back as 500 years or something and yet surprises always show up.
You have to understand that you have 256 6th Great Grand Parents, 512 7th Great Grandparents, and 1024 8th Great Grand Parents, and it continues to double after that. It’s impossible that people have been able to account for each one. You could have 1 out of the 1024 8th great grandparents that was originally from Ghana Kingdom that was sold south after a war with Mali, somehow made it over to Dahomey, gained his freedom, became a trader, traded goods with neighboring Oyo, took a Yoruba wife, had a daughter, then she marries a Yoruba man, all the way down to you.
You won’t know that until you test. Also, it helps improve the results for Africans around the diaspora as our results fluctuate because of the small numbers of continentals that are testing.
Let’s say I descend from a Yoruba man. If actual Yorubas in Nigeria don’t test, then my results may come up as being from Benin, because the Dahomeys are close in relation to the Yorubas and so the Dahomey/Fon population in Benin would be the closest population my Yoruba DNA matches to due to the small number of Yorubas that actually test. The more continentals that test, the better the Diasporas results will be.