Welcome Valentina!
You are so right, often the green city spaces are so heavily used that they are not good to forage from. For the West, I actually stay away from Westerpark for foraging – although it is a lovely place to be – as I feel there is plenty of soil contamination still. I may be wrong.. I suggest slightly out of the center if you want to forage. There is that extension of Westerpark, where it shoots off and becomes quite wild. Heading towards Halfweg I think. Here is a link to a map of some green spaces in Amsterdam – the bigger spaces. I think it is the area from Westerpark towards Sloterdjik.. Along the railway is not good so bear that in mind. How about Noord? Lots of space there. And the space I mean in Westerpark has a small buurtboerderij (tiny farm). I hope this helps a little.
https://www.amsterdam.nl/en/policy/policy-green-space/
Let me know if the second link is no good..
Also, I recommend getting to know the small green gardening areas in neighbourhoods. Some old friends of mine run a herb garden just after Paridiso on the Weteringschaanscircuit. They grow gorgeous herbs and I am sure they have some suggestions.