I’m not quite sure what this experiment proves.
But I tried it.
Unfortunately, the flicker did not disappear.
But you can see that after turning off the MCU flicker has changed the frequency
(the first 10 seconds with the MCU,
after 10 seconds the MCU is off)
20 sec flicker
It proves that the MCU and WLED have little or nothing to do with the flicker you are seeing.
Look around this forum, there’s plenty of examples of people filming effects and projects in action without the effects you’re seeing.
I would suggest you re-examine your assumptions about how your camera and/or filming should work with these devices.
You haven’t documented your setup or even what you’re trying to achieve.
What is your actual setup that will produce these effects?
How is it not related?
What about the change in flicker frequency after the MCU is turned off?
You’ve seen it.
In the final version - it is an ordinary LED tube, like a floor lamp
It’s not related because the effect appears whether the MCU is attached or not.
The fact that the effect changes when the MCU is attached is an addition, not a basis for the effect.
The videos posted don’t give any frame of reference as to how far the camera is from the light source, what frame rate you are using, what aperture, etc,etc,etc.
It almost looks like you’re trying to zoom in on 5 individual pixels and hope to NOT see RGB separation within the LED.
One further thought, try to go battery powered to eliminate possible power supply hum/interference.
Of course I tried through the powerbank, but there was no difference.
Please understand me correctly: I’m not arguing, I’m just trying to find a solution based on logical inferences.
There will always be flicker. You need to adjust your camera correctly to try to combat it.
Hallelujah! 
What type of illumination is in the room where the LED strip is being filmed? My apologies for not viewing any of the linked videos.
If the room is lit by LED bulbs, turn them off.
If the room is lit only by the LED strips, temporarily change the strip length in the WLED configuration, say 1/2 of the actual length. Film and test for flicker. If it gets better or worse, you might be able to find a sweet spot where the LED strip update rate is high or low enough for the current camera shutter speed.
If you have a high speed camera available (a newer high end iPhone can do higher-than-normal-speed if you do not), try that out to see if you can find a good frame without flicker.
Or, pump up the brightness and slow down the shutter. (That should get rid of the flicker while ruining the photo’s quality)