Thank you very much for the instructions fender3x
I bought all the recommended tools, let's see if I can handle it...

I tried to make the neck as flat as possible, but what really makes me nervous, that after the 5th fret the fretboard/neck makes a curve and has about 1 mm difference at the nut, compared to the 5th fret...I think this is visible in the pictures very well...

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So in my opinion I would have to sand the whole fretboard 1 mm down, from 22nd till the 5th or 3rd fret...to kind of get a flat surface...
That sounds horrible in my mind

And as to the light test...I used a metal ruler, which seems to be very straight...so, it starts with a glimpse of light at the 22nd fret...then 17th fret...then 15th - 9th...and then between the 5th and 3rd fret till the end/nut...So, those seem to be waves on the fretboard...I can imagine, that this would not be an issue with frets, but on a fretless, this seems unplayable to me...

For the sanding, I am not sure how thick the fretboard is...with all this information, is it possible to solve the problem at all?

And I noticed some traces on the fretboard, I think it comes from the strings...hope it can be seen in the picture...That is not normal, is it? I mean I barely "played" it

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So, even after some builds...still did not check every detail before starting the build :/