To answer the question: No, USF does not need non doctrinal heavy mobile artillery. They have 3 non-doc artillery options, with 2 really good doctrinal units (and one "meh" unit).
Essentially you are asking to give them a non-dock preist because you don't want to have to pick Infantry Company in every match, and i get that but it's just not a good reason.
I mean, team games are TEAM games, if you don't want to use Priest, ask your teammates to do it, so you can do something different, and if they don't want to cooperate? get some better teammates.
at this point 3v3 and 4v4 would require their own set of units and rules, but its too late for that
Honestly 4vs4 is as good as it ever was right now, fairly well balanced and fun to play, and with more viable options than ever. It's just people either don't want to adhere to meta, or don't understand that its a TEAM game mode, and you can't do everything by yourself.