i didnt read all posts. Finally i understanded, :-)
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Then again - Your SMPS is nice thing , and - in my opinion - is not "compatible" in terms of quality and elegance - with 1_Resistor_ZCD.
I prefer optoisolator here.
Non-isolated ZCD means You will connect micro's GND or VCC to Neutral or Live wire. Last case needs care.
Thalking about bad quality of mains , You must take some prevantive actions in hardware and software , and check it against worst case.
Here i think 2-optoisolator circuit ,proposed by K.Kaas, is good starting point. And many IF-THEN-ELSE in software - against false signals.


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3 wire is supercomfortable environment :-) .

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500 W is nothing special .

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"8 steps of brightness" - i suppose You will come to user selectable brightnes, which means more than 8 steps . Or 2 differet modes - with 8 steps and with smooth kontrol.
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Single transistor ZCD , or single optoisolator ZCD - difference is only in safety , some cost , and simplicity/complexity in software .

If You prefer, let talk about transistor, without isolation.
250VAC 1Mohm gives 0.044W , so resistor size is determined only by 250V requirement. One end of resistor is connected to Live, other to small MOSFET's gate. MOSFET's source is connected to micro's GND , which is tied to Neutral.Between gate and source will be one zener diode 5..6V ,with anode to GND.Drain is connected to micro. This is my proposal :-) .

best regards