I have a Buick Lesabre with similar issues. I start slowly from stops because sometimes it slips, and then bangs into gear. It’ll also slip a bit when shifting between gears, then start to shift hard. I’ve changed the fluid like you, and didn’t notice a ton of material in the pan. If anything, my latest fluid change seems to have made the issue worse. I suspect in my case the trans won’t last that much longer. If I wanted to keep the vehicle, I’d probably take it to a transmission shop, but I feel certain they’d want to rebuild it and I don’t want to invest the money on my 14 year old car with 170k miles. If you plan to keep the Trailblazer, you might take it to a good transmission shop, describe the problem, let them check for any stored (hidden) transmission fault codes, and make a recommendation. It’s possible they might recommend replacing a shift solenoid or something and you might get out of this fairly cheap. But you have to find a good transmission shop. In my case, I’ve had bad experiences with the 2 closest transmission shops in the past so I’m just going to baby the car until it’s no longer practical to drive. I did drive it 15 miles to work this AM (it does fine on the highway)!