Yes keep on at them. The ones that suffer unplayable lag but don’t go through the support process have nothing to complain about. Eliminating possible causes is what it’s about.
Did they get you to send them diagnostic data recorded during gameplay?
If you and Blizz eliminate all other possibilities for cause of lag, looks like too many players during large battles for some reason affects you worse than others.
At least the battles are typically over quick, are not on all the time, and don’t always even start.