A few tips on physics;
Oversteer can't be fixed in any way except to make the car properly. Adjusting steering ratio is a bad sign and will not affect balance at all.
Front geometry is surely not set at design height, so the roll center is very low underground. I suggest designing suspensions at the unladen height with no driver and fuel.
Rear suspension, just look at my 106 Rallye S2 from Daniel or classic Mini from Pessio for a trailing arm and my E30 on RD for a semi-trailing arm. I think it's trailing IRL in the Fiat.
To find the center point of the suspension:
Open "telemetry" app and find the suspension travel section
1l fuel
Tap the gas to get the car to settle
Suspension travel will give your "rod_length" value for the 0 point in millimeters ie: 100 = 0.100
Put in that rod_length
Measure again
Beware if the load on the axle or the springrate changes, it will change too.
About springrates:
The ride frequencies are like 0.9 and 1.1hz, wheelrate around 5000, I doubt it. Car designers avoid 1hz frequency due to nausea it causes. More common would be slightly higher, like 1.2 and 1.4 for example.
I'm just mainly curious how you got those values. Manual? Guessing?
If I was looking at the right car, it's front strut and rear trailing arm with coil spring.
For a strut front the MR will be commonly quite high so springrate will remain quite high. Rear motion ratio is very low, maybe around 0.3 - 0.4 for the spring, so I would expect the rear springrate to be much higher than the front, but end up potentially even lower in an economy FWD car. The load split of the car will turn it into a rear-biased frequency setup usually.
Some doubt on the rollbars too. They're 2500/2500 right now, which is possible (NSX is like 1500 / 2500) but I would doubt it in some cases. To make sure calculate it.
Here is a calculator:
Anti-Roll Bar Stiffness Calculator & Anti-Roll Bar Shop - First in the UK! Work out bar stiffness then buy the best upgrade online .
balancemotorsport.co.uk
Fill in the ARB dimensions from the manual, or do a visual estimation (Can be hard to find pics for a car like this) or measure it yourself. Remember to apply motion ratio to the bar as well. Link and bar bushings generally also remove rate from rollbars during nominal travel, usually the rate ends up 0.3 - 0.6x from the raw rate. I use 0.4 for most Shore65 rubber bushings.
However sometimes they do end up very low and close together. 2000 DC2 GS-R:
FRONT=3388.7143206
REAR=2280.04769319
;F ARB diameter = 24.0 Solid
;F ARB MR = 0.3819^2
;F ARB Droplink angle = deg =
;F ARB dimension X = 215
;F ARB dimension Y = 805
;F ARB dimension Z = 265
;F ARB bushing effect = 0.4
;R ARB diameter = 14.0 Solid
;R ARB MR JDM, USDM Type R/SiR = 0.5138^2
;R ARB MR USDM non-R Integra = 0.5216^2
;R ARB Droplink angle = deg =
;F ARB dimension X = 122
;F ARB dimension Y = 950
;F ARB dimension Z = 123
;R ARB bushing effect = 0.4
More sporty models will usually have a lot of rear bias in FWD cars. 1998 Type-R.
FRONT=4987.23649016
REAR=20144.9565249
;F ARB diameter = 25.0 Solid
;F ARB MR = 0.3819^2
;F ARB Droplink angle = deg =
;F ARB dimension X = 215
;F ARB dimension Y = 805
;F ARB dimension Z = 265
;F ARB bushing effect = 0.5
;R ARB diameter = 23.0 Solid
;R ARB MR JDM, USDM Type R/SiR = 0.5138^2
;R ARB MR USDM non-R Integra = 0.5216^2
;R ARB Droplink angle = deg =
;F ARB dimension X = 122
;F ARB dimension Y = 950
;F ARB dimension Z = 123
;R ARB bushing effect = 0.5
Bumpstops as well, some doubt. Did you accidentally put in the spring rates to them? They look like sensible wheelrates to me. Bumpstops/packers in normal AC should commonly be the nominal portion of the beginning of travel. However you can use them for whatever spring you want. Beware STRUT has extremely stiff bumpstop_up so you will need to use the packer for the shaft-mounted bumpstop.