[2020] Am I missing out on the game if I only play duo with my girlfriend in a casual way?

It seems lot of people play (I'm guessing random joining) with teams of 4, and I'll admit that it does seem like the 4 classes have great synergy. I played Engineer a few times before but now that she's in the game she picked Engineer and I'm Driller. I haven't promoted anyone yet but I think we will reach promotion soon. But even then we're only playing as two classes, not 4.

So I'm thinking sometimes that maybe we're very unefficient, as in if we're only two we don't get a lot of xp or materials. Although, I'm honestly not thrilled to play with other people seeing as it can go badly with uncooperative mates and of course less intimacy than with just her.

Are there any tactics we should use to help compensate the lack of teammates? Should we be rotating classes? Thanks for the answers in advance.