First, my experience is that inflation pressure doesn't have as much effect on evenness of wear in steel belted radial tires compared to other things like position on the vehicle. Bias tires were more sensitive, which is where that myth came from. It's just not as true for modern tires.
So with regard to the OP's situation, the general advice is that this type of wear comes from lots of cornering - hilly terrain or city driving. But that doesn't seem to be the case for the OP's.
Grasping at straws, I am aware that suspensions are compromises and that you can either have one that doesn't change camber or doesn't change track width (the width between the contact patches), but you can't have both. The third possibility is neither. So I wonder if that Subaru's suspension is one where the compromise is on varying camber and not varying track width. That might explain the two sided wear.
If that's the case, unless the OP wants to rearrange the suspension pickup points - which involves redesigning things, then welding - there isn't anything to be done.