Wow, I tried a IRF730 MOSFET as switching device and it is much better now...!
It can drive any of my previous loads perfectly without getting even warm.
But being such an ideal and fast switch, now at the lowest PWM level my LED bulbs
are starting to shine at ~40%...
The impulse is so short and even so they are quite bright. :-o
So as the startup has a lot of current, the dimming doesn't produce a linear
control of the lightness but follows exactly that current pattern in time.
So I guess the next step is to build this zero-cross synced 555 based trailing edge AC dimmer.
My last chance is that starting the "turn on" on a low voltage may help to smoothly dim the LED bulbs at lower levels.
I attached my design and the predicted waveforms with the lowest and mid level dimming. In spice it works fine.
A question: I used a trick to have a simple zero cross detector which provide a short pulse
but I'am not sure: leading the mains via a 1n capacitor can kill the transistor base-emitter junction
or as it's a small pulse which starts at zero so it won't do any harm?
(I designed a 2nd version of that part using a resistor + zener combo to protect the transistor if it blows in reality...
)