I had a similar issue in my 2018. I ended up putting the hard rubber spring spacers in the coils in the back. One spacer on each side. The spacers I used are the ones that go inside the coils, like what is used in NASCAR to tighten up the suspension.
I too drove very spiritedly, and typically had around 200 pounds of stuff in the back between my sub, my tools, and my pack. I live close to a major toll road here in Texas that has the highest speed limit in the country, 85 mph. Typically I would set the cruise to 95 mph, and get passed!! Corners were fun in the car too. The Niro rotates REALLY good with the throttle.