Hi Joe,
I bought my 2009 RX450h a few years ago, shortly afterwards I felt exactly like you, disappointed with the ride quality but otherwise very nice. I changed the rear shocks, it made no difference, after a leaky front shock I also replaced both those. changing them made no real difference to ride quality.
I tow a caravan and put rubber spring assisters in the rear coil springs. I think that has improved the roll at the rear a bit.
The main reason I have not changed the car is that it has a tow bar and is fine for towing, it's immune from ULEZ problems, I don't want a diesel due to previous DPF nightmare. Cant really afford a newer more expensive model. I only drive about 7000 miles per year. I like the comfort and finish and space.
Driving on fast A roads or motorways is good. It's pretty quick and grips well round corners. But all I wanted was a smooth comfortable ride!
I have just got used to it as it is. Not sure what else to buy !