How to convert R,G,B,W signal to 1 wire in LED strips
All RGB/RGBWW/CCT/etc constant voltage strips are wired the same: common anode (ie, &quot;Plus&quot;) and one cathode per color. The controller connects each cathode to ground with a FET to light the corresponding LEDs.
Brightness is usually modulated with PWM. It would be very surprising to find a switching constant current inside these controllers.
So, if you want to use this to control WS2812B:
First, measure the voltage between the controller's ground and the outputs while playing with the controls, or just open it to check the outputs are just simple FETs to ground.
Then:

The FET on the left is inside the controller. You add the components on the right to filter the PWM waveform into a voltage that can be acquired by the ADC. You can also remove the capacitor and use one timer in capture mode per channel to acquire the duty cycle, but that's a bit more software.
Then you need to write code to translate these values into WS2812B protocol and drive them from your micro.
Note WS2813B are quite inefficient and expensive compared to standard 24V RGB strips, so using WS2812B is only justified if you need to address each LED individually.
You could also check if the controller can be powered by 5V instead of 12V. If it works, then you can use the same 5V power supply for everything.
EDIT
Note &quot;MicroVCC&quot; on the schematic is the VCC of your Micro, ie the max voltage the ADC input will tolerate. If the micro runs on 3V3 through a 5V LDO from a 5V supply, then what I mean on the schematic is of course 3V3.
Also the output is inverted. LEDs OFF will be max ADC value, LEDs ON (thus MOSFET ON) will be ADC close to zero.