QGIS 3: Symbology with Geometry Generator - Draw One Convex Hull For All Features With Same Attribute

I have a layer named "Trails" with an attribute "Trail ID", which contains unique, consecutive Feature/Trail IDs and an attribute "Schwierigkeit" (trail difficulty). For all features with the value "black" for the attribute "Schwierigkeit" I want to render one convex hull. The below code will select the last element of all the features in the layer and apply the code for drawing the polygon only once. The first

array_foreach()

in the code will create an array of all features (series generated from 1 to feature count number). Over this array, the second array_foreach() will apply the geometry function on each element that meets the condition of the

array_filter()

function. The

collect_geometries()

function finally puts all those single geometries within the resulting array into one multiline geometry, for which I then draw the hull. The purpose of this procedure, is to check if the trails in my dataset show a spatial aggregation according to their trail difficulty..

if($id = maximum($id),
convex_hull(
collect_geometries(
with_variable('my_arr', 
array_foreach(
generate_series(1,  layer_property( 'trails', 'feature_count'), 1),
get_feature('trails', 'Trail ID', @element)
),
array_foreach(
array_filter(@my_arr, attribute(@element, 'Schwierigkeit')='black') , geometry(@element)))))
, NULL)