Axle suspension setting

Because my son pointed out the poor ride on rear passenger's seat of BC7 SUBARU Legacy LSi US model last time, I have a planning to replace rear suspension into another setting, putting back the original suspension coil springs with harder shock absorbers or another combinations, all the parts would be applied under the condition of the use of not third parties but genuine parts only. I would be at a loss what to choose, because there are many combinations at all. All the suspension arm bushes should be loosened all together and re-tightened at Zero gravity condition. The strut upper mounts especially for the rear axle should be deformable enough to consume the shock from the ground, involving some barrier of particularity, tapered suspension springs applied only for initial model of BC7, so different rear strut upper mount is set for initial production model. If no stock in maker's parts depot, some adequate rear spring rate should be applied because of the compatibility.
Lower arm lever ratio has been changed in order to make use of the deformability of the trailing arm hinge compliance bush contributing comfortable ride. So some harder spring is used intentionally for recent SUBARU models. But remember here that  too much application of compliance here invites sometimes unpleasant rickety feeling leading to anxiety, lack of stability in short. Anyway, change of suspension components would make it different one as I did several years ago. I did it from comfort American design to sporty European on, now I do the opposite. Just trying on a whim is easy for everyone, but it would be difficult to establish and finish the work is above words, because what I'm going to do is nothing but a professional's work like an engineers in some researching and developing division of an automobile production company. As I'm keeping several parts for the trials in my villa so I'll do it in very near future.