Hi Mark
You could stiffen up the rear end a bit, by mounting bracket’s from the rear axle bearing mount to the seat, we used to have to do that when we raced in formula A 100cc in the late 80’ties early 90’ties, to prevent the rear end from hopping, sometimes we ran up to 4 of these brackets, 2 on each side. Moving from a 30 mm axle to a 40 mm one helped a bit.
But with a soft chassis you should be fast in the rain :)
Best regards Matthias