Dark GM Paint is a bugger. I used to have similar issues with the Dark Blue GM paint.
Below is what I changed, listed in the order of what I feel helped the most. Not fool proof and I'm sure others may disagree. Just what has helped me.
1. Use 4-5 different mitts when washing and swap out the 'rinse' bucket often. Grit guard or not, particles get trapped in mitts regardless of swirling and rubbing on the guard.
2. Use 3-4 drying towels kept in plastic bags until ready to use. If there is any draft/wind at all where you are washing, regardless of how good and quick you washed the vehicle, new scratch capable particles will find there way to it quickly. I also think they come from the water we tend to rinse with.
3. Although I love the feel of big fluffy wool mitts, and they advertise to trap particles "deep in the fibers to protect the paint" I believe the particles get so "trapped" in wool mitts that they don't always wash out and contribute to what you are experiencing. No matter how "deep in the fibers" the particles are, they move to the surface when washing with them. I have switched to all Microfiber mitts. Microfiber fingers are shallower and easier to notice the trapped particles. They also give more feedback in my opinion. I know it's super geeky, but it has seemed to help.
4.I tend to go "against conventional wisdom" when washing. I wash the dirtiest sections of the car/truck first always wiping 'down', then move up to the top and wash back to the bottom. Swapping mitts out often.
This photo is a poor attempt to illustrate. I divide the vehicle up in 3rds. I wash the bottom (green) third first, swapping mitts out two or three times. Then do the top (purple) section with a new mitt or two. Finalizing with another new mitt or two on the middle (blue) section. The arrows are meant to illustrate the direction I wipe.
Probably has little to do with anything, but I like to think I came up with a new idea to change the car cleaning world forever (joking)