I’d go this route or GSXR rear personally.
Both cheaper options with proven results.
Before either of those though I’d check 3 things. Chain tension, looser is better. Tire pressure, lower is better. And your body position. Get your weight over the front not on the rear. Then there’s less fat boy on rear. Most of your weight should be over the tank.
Not saying the suspension isn’t the problem, just saying those three things almost always are the real problem when street riders get a taste of the track then start pushing themselves and paying attention to what the bike is doing.
Going to a Q3 or 4 tire will help too