I have struggled with the *exact* same problems
1/2" screws at 5' are really hard to get straight. Even with a "straight" screw, by nature they will begin to whip at high speeds. Over the 5' span, gravity alone causes the little 1/2" screw to sag a little. I switched to 8tpi 1/2" screws on one machine, and it's better since they don't have to turn as fast, but I still think I'd have been better off with 5/8" 6tpi. The problem with that is, that requires all new bearings and motor couplers. I was also concerned that the motors may have a hard time dealing with the extra mass on fast, tiny movements. Maybe someone else has experience with this.
I also made some of those dampers, and aside from making a LOT of noise, they did little, if anything to help. I took them off after a few hours of messing with them. They may help for other problems like resonance, but not for my problems.
On the machine that I'm talking about, I'm using those plastic anti-backlash nuts, and those things can be finicky. I could feel some small bumps / burrs on the acme screw, so I took some fine sandpaper and ran the screws back and forth with the sandpaper wrapped around it until things felt a bit smoother. After that I cleaned everything up really well, lubricated it again, and it's been better. I still run it slower than I'd like though. I decided I'd rather have my projects come out decent rather than wrecking every other thing I cut. Steppers have a lot of torque when they turn slower, but it quickly fades as they increase in RPM.