Thank you for the comments so far:
@snitcythedog: I assume you refer to the frozen lake? I had a much darker blue first, but then added the whitish stripes to depict deeper layers of ice (or whatever produces these white stripes

). I could see that a darker centre would give an even more realistic impression, especially if the lake would be bigger.
@Major_Gilbear: Is the blotchy appearance to you all over the piece or only in some sections? The pigment I mixed in the wash had some bigger pieces in it which gave it a less subtle appearance. I did contemplate a drybrush with light sand colour, but also do not want to add too much contrast as this will make stuff look less realistic. Some of the colour differences are due to the different materials used. The sand takes paint and wash easily, while the paste comes out a tad lighter.
@cacofold: I appplied a top coat of acrylic medium. PVA works also like a spell. On foamboard, MDF and plolystyrene the crackle paste does not lift after the application. It will lift without. It can be played on without issues and the surface with the top coat seems resilient to normal gaming use. I did not mix paint with the paste, but gave it a spray of sand colour after it was fully dry, followed by a wash. I am not sure what will happen if you mix it with paint. It will affect the cracking, however, to soem extend.
@Wirelizard, @obsidian3d, @FramFramson, @jimbibbly: Thank you very much for your comments, much appreciated.