BradtheRad: Forgive me, but I seem to be missing the point here. Can you please elaborate your solution? How can I change the adjustment pin voltage with a microcontroller? I would need some sort of DAC for that, but in this case I might as well just supply the DAC voltage directly to the LED's instead. No need for PWM when I have an option to adjust voltage directly, riiiiiight?
I'm sorry, I did not understand whether you want to dim/brighten all chains together simultaneously with one adjustment, or dim/brighten each led chain independent of other chains.
I guess it depends on how many jobs your microcontroller is able to do, and whether it can supply the required voltage, and required amperage.
I have no direct experience with microcontrollers, but I understand they have several output pins (On/Off). You may find it is possible to program it to pwm 6 mosfets independently. Each output pin would go high and low many times per second.
Example, all outputs go high simultaneously. Then the program pulls each output low after a designated duty cycle.
If you find you need middlemen devices, then it is convenient to implement analog settings through each one. So in that sense you're correct about needing to send a DAC voltage from the output pins. A different voltage to each middleman device.
The middleman devices might be sample-and-hold circuits. The output could adjust the duty cycle coming from a 555 timer IC, in the same manner shown in my post #8.
I cannot say which method requires more effort, compared to which method needs fewer parts, etc.