Custom REST Resource POST problem
Looks like you've stumbled across the same stumbling block as I have a couple of times. Try with this piece of code - note the create entry after canonical: 

 *   uri_paths = {
 *     "canonical" = "/foolzz_rest_api/profile/{uid}",
 *     "https://www.drupal.org/link-relations/create" = "/foolzz_rest_api/profile/{uid}"
 *   }
 * )


Read more at: 
https://www.drupal.org/node/2811757
and
https://www.drupal.org/docs/8/api/restful-web-services-api/restful-web-services-api-overview

Oh and for the post request, I'd use ModifiedResourceResponse instead of fiddling with cache headers by hand (I learned about the existence of that from Drupal Core's EntityResource)