OK I’m like the sport chassis ambassador ha so let me get on this. I’d like to understand what you feel as “front heavy” not picking you apart, just want to see if what you are looking for will be addressed and by what. I depends on what you like of course. IMO the sport chassis is tuned very neutral, the front suspension is double arm so grip is good without the ability to easily add camber. But I’ve come from sport compacts, It’s not like a Subaru. By default I think you need the sport chassis, that does depend on what you are able to turn up used and what the premium is though. You can read all my thoughts on why: PSA – Sport Chassis
Anything can be done…what effort to get there though. As you say the front is super easy with simple coil over shock. There aren’t any good options IMO for aftermarket so the factory sport spring and perhaps the damper as well would be best there. The rear again the shock is a shock. I am seeing the B6 for the front is showing up now, but no sign of the rears. I suspect the dimensions are the same as the S/V90 B6 already on the market, but the tuning is different so different part number. The B6 is a monotube so could always be re-valved by your favorite shock expert. I would wonder about the out of the box tuning, it’s a single part that’s a “sport” line but also made to work with both the “dynamic” and the “sport” chassis springs. Finally you can go to the KW V3 coilovers, which are 3-way adjustable coilovers (minus rear spring) so you could tune to your heart’s content…that might take a lot of fiddling.
Ahh now let’s talk about that rear spring…it’s possible, just not known. I might have done the most research…the design of the rear leaf has a few factors, the spring itself, the inner chassis mounts and the outer mounts/spacers. There are a number of variants of each. The Volvo service manual explains that the spring itself is varied of course, as well as the chassis mounts and the outer mounts which vary the rate of a given spring. The spacers are calculated/chosen by computer at the factory for each car to adjust to the correct ride height. It’s a bit to decipher if you go down that rabbit hole. I have seen some overlap between part numbers such as an XC90 rear spring that matches a Polestar V60. It’s all there just might take you a lot of research and ordering parts unless you can get to a Volvo engineer to help you.
As to adding coil springs in the rear mentioned…again possible as always! The subframe in the air spring cars is different (it has upper seats for the air springs), and those vary at least suspension geometry and diff mounting (ERAD or mechanical) for different models.
What is your source for the spring rates? I’m just a little doubtful as the “spring rate” is not the wheel rate on this car, there is motion ratio due to leverage front and rear. Nobody has dyno’d the springs that I know but perpahs that has changed. I took measurements and did a calculation, estimate the front spring rate at 425lbs (~225lb-in wheel rate) and even harder to try to estimate the rear composite leaf but I got 640lbs (~241 lbs-in wheel rate) on my 2019 S60 T6 R-d with the sport chassis option. It confirms with my experience modifying suspensions, feels like a ~250lb sprung car, that’s just my feeling though, but I think I am not too far off. The rear sway bar is common across all non-CC models 22mm. I did find a 23mm version for the V90 which I swapped into my car. The front 26mm sway on the sport chassis is shared with the Polestar model. It lends a lot of roll stiffness to the car, which gives a very satisfying response. Sport chassis sits about 1/2″ lower than the dynamic.
This is all I know, hope it helps. The far easiest way to approach this is find a sport chassis car!