I only lag when I’m in a full party. Why?

I’ve done everything under the sun to improve my connectivity for Apex Legends. Bought an expensive gaming router which I opened all the ports for EA and Apex, and allowed for IPv4 and IPv6 to run. I have the fastest speeds available locally at 500 + down and 20 + up that I pay a lot of money for. I bought an expensive modem with multiple channels so I’m not interfered by neighbors traffic. I’m hardwired into the router but I still have to turn off all wi fi connections for smoother game play, and when I hardwire into the modem every server has packet loss. I’ve spoke with EA help on multiple occasions, ran a trace route, and nothing showed up that they could see. I reduced my video from 4K to 720p just to increase frame rate. I’ve even gone as far as turning off usage sharing in the hopes that it wouldn’t bog down my connection for whatever reason.

With all this being said, gaming is supposed to be a community where you can hang out with friends and have an enjoyable experience, which I’m not able to do. I quit playing with friends and started solo queuing for months because I couldn’t play a match with a party without causing everyone else to lag too. 

Please help. I’ve spent over a year and hundreds of dollars trying to fix this issue. I just want to play with my friends and not feel guilty for making them lag and then they leave☹️