I've only played around with R&T on the F32, but I can't imagine it's much different than on your F87. The spring adjuster is what determines the ride height. When the car is on the ground and loaded the spring is what supports the car, so lengthening/shortening that assembly is what controls the height.
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Preload, for this situation, is set by the shock length as the shock is what’s maintaining tension on the spring while in full droop.
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When you get to really high spring rates (like in this kit) the weight of the car doesn't compress the springs very much (less than an inch in your case), thus Ohlins is probably using the damper/shock length to limit the amount of droop (suspension unloading) per their statement to you. If you set the rear spring height adjuster to "all the way up" or to the lowest ride height, and disconnect the shock, I'll bet the spring will be loose with the car up in the air. In a situation where the damper/shock body isn't adjusted it's possible for the spring to become loose and unseated if droop isn't controlled while driving. Other companies combat this by either using a progressive rate spring (like Bilstein), where the low spring rate coils only come into play during droop, or by using helper springs, which also keep the main spring seated during droop events.
The spring rates don't look terrible to me, but I'd recommend lowering the front spring rate to have a higher rear ride frequency over the front. This will help the car settle faster over road features and make it feel more stable. The F32 front rate of 60N/mm would probably work well for you (for reference that kit uses a 160N/mm rear spring). Too many people only see the "scary" high rear spring rate number and think that's the one they need to lower.
When I was dialing in my friend's F32 R&T dampening the recommended Ohlins settings were too overdamped. That can cause the ride to be too stiff and harsh, per what you're experiencing. Part of that is because the dampers are rebound biased, so they don't return to neutral height as fast as they get compressed. When that happens you get pulled into the bump stops, which add additional spring rate and pop you back off. It also prevents the wheel from recovering as quickly over features like track curbing so you have less traction overall. For my friend's car we paid attention to how harsh each end of the car felt going over features at speed, and then turned down the dampening accordingly. In his case, the rear felt harsher than the front, so we lowered that by a greater amount. This drastically smoothed out his ride, and he was able to have a more comfortable daily driver, as well as use all the track at the track since the suspension was better able to handle the curbing and maintain traction. At our last outing two weeks ago a lot of other drivers at the track were commenting on how the curbing at certain turns was harsher than in prior years. We just laughed because our suspension were dialed in and able to handle those features smoothly, thus allowing us to attack them with greater speed. Stiff isn't fast, but smooth and controlled is.