Despite what some other people might say, it's only a myth that t2s make the bonuses better. I barely have any t2s and Ive gotten Legendaries and basics. It's all just luck.
Here is the deal. Bringing more t2 or at least maxed 6* characters improve the chance of good reward . That is the good news.
The bad new reward is not only affected by your team but also your teammates team . So if you bring 3 t2 and both of them bring several 1-5* they will screw the reward for themself and you and you 're chance to get white boxe dramatically increase.
I think someone threw down some calculations from their experiences that showed the higher the level of team members the higher the chances for better loot boxes. If you search this sub in the last week you should find it. I’m sorry I don’t have a direct link.