Thunders work fine with barrel bushings. The boardside bushing is barely conical anyhow.
To keep geometry correct while playing with bushings of lower height I use flat washers under the boarside cupped washer to make up any difference. With a bushing to tall you can eliminate the boardside washer.
So that my 35lb son can turn on his Thunders I use Ace roadside cones as the boardside bottom bushing with a couple of flat washers for height and Indy soft red roadside without any washers on top.
When it comes to bushings, formula, height, shape, durometer, washers, lubrication are all factors to be played with.
I find stock thunders to be very stable with a good turn that can be muscled into. I prefer less stability the more I skate and have been swapping cupped washers for flat ones with stock bushings.