This is starting to sound really ugly.
My experience with this was a different type of RGBW, 5V pre-soldered with 1cm boards on 10cm wires.
Worked great (I thought) except for a few flicker issues with particular colour settings. Blamed it on my Arduino code forced to run on an ATTiny85 MCU (very small, 512K RAM). Didn’t need effects so ignored the flicker.
Jump ahead 2Yrs, someone else complains of similar issues with a SK6812 RBGW string that looks suspiciously like mine. They further diagnose something I never thought of: the onboard 75R0 resistor.
Normally that resistor is inline with the Do pin, giving some noise tolerance. On these boards it was inline with the chip Vcc pin!
That means power through the chip was limited by a 75 ohm resistor, messing up the supply voltage as brightness increased. It caused all the weird effects noted above. The final solution on my strings was to bypass/remove the resistor completely. You can see some more details here: SK6812 RGBW unstable - #19 by divsys
Your strings are a little more complex as there is only 1 “smart” LED in each block of 6. I’m willing to bet if you spent some time with a multimeter, you’d find a resistor inline between the 24V power line and the VCC to the smart LED.
Unfortunately, I don’t have a solution for you as I don’t have any specs on the smart LED in your strips. I don’t know if it can live connected directly to 24V or not. I suppose you could try and sacrifice I block of 6 and try it out. Just be prepared for smoke…