Extremely Rough Ride on IS350 w/ Lowering Springs
I recently purchased a 2006 IS350 with roughly 100K KM on the odo. The car came with F-Sport springs installed, rolling on 18" 3IS F-Sport wheels mounted on Bridgestone Potenza tires (225/40, 255/40). With that setup my IS was already riding extremely rough, while I do understand that the IS350 is a sport sedan with stiffer suspension compared to other luxury sedans, I feel like it rides worse than my 2008 Civic Si that's lowered around 2" with stretched tires. Mainly when I hit a rough patch of road, or cracks/crevices in the road, it almost feels like I'm gonna lose a part of the wheel. Shocks are okay the ride isn't bouncy at all, it just seems to make a lot of noise in the interior and a lot of creaking inside the interior when driving over rough roads, and the ride is extremely harsh.
Few months later, after accepting the fact that this is how it is, I went ahead and installed H&R Sport Springs to improve the stance and handling of the car. I also switched out the summer tires for some Falken Eurowinters (225/40, 245/40). Did an alignment, pumped 35/37 PSI into the tires, and I'm sure the springs have already settled (driven almost 5,000km on them already). At this point the car simply rides like a go-kart, it rides so bad that I simply do not want to take any passengers or they might get car sick. I understand that lowering the car any given amount I will sacrifice ride quality, but forum posts do state that the H&R's shouldn't be too much of a difference from stock, at least not to the point where I feel like I've made a terrible financial decision in purchasing the IS350.
For reference, when I drive over potholes, crevices, highway bumps in my Civic, it's stiff but the car seems to absorb much of the shock, and it really goes unnoticed. Driving over potholes and crevices in my IS, I can feel it in the interior and my door panels and seats will creak and cause a lot of noise.
Is there anything I can do at this point?
Pic of my IS for reference.