Today I tested the N-FET construction with small pieces of LED strips. With all four PWM channels, all of them have around 500Hz an can have any duty cycle. The main result of this test is, that I have to choose other values for the pull-up resistors. I choose 3k3 for 3V and 4k7 for 12V.

For the tests i used a breakboard from al1. https://hackaday.io/project/6768-atmega8-breakout