Tholeone part in the video you briefly touched on was the reason for the 6v6 format, and i think the biggest factor by far you didnt mention was that a voicechat with more than 6 people in it is just a mess. highlander comms are gibberish compared to 6s because 9 people talking at once just doesnt work lol. 6 people is pretty much the max and most other competitive shooters opt for 5.
honestly i didnt ever consider that but thats a great point. i might make a followup video or something because theres a few things that I feel i didnt elaborate on enough. like why specifically 5cp being dynamic makes it more fun compared to other gamemodes