Alright, after making some small adjustments to the layout, it's now fixed and already went through the cutter. Instead of sticking the adhesive foil to the glass of the picture frame as I previously planned, I applied it on a simple sheet of paper. This way I have absolutely no struggle with dust particles or air bubbles and the paper is also functioning as a diffusor for the LED light.

The WS2812 stips are already applied to the back board and I also added a completely hidden photoresistor behind the black foil for automatic brightness adjustment. (Fortunately, the film is nearly but not 100% opaque).

A 3D printed spacer grid ensures a defined illumination distance and blocks nearby LEDs. A first style-check of the layout inside the frame reveals... it could become a beauty :D  Can't wait to see it light up.

Next steps include finishing the electronics and having some fun with the code editor.