qgis - Displaying (or filtering) features in one layer based on features selected in another

I have points in a layer "Site-Centroids".

I generate a lot of isochrones (polygons) around them, into "Isochrone_Site-Centroids".

With all the isochrones displayed, the map is too busy.

What I want is that no isochrones are initially displayed.

Only when the points from the original "Site-Centroids" are selected, do I want the corresponding isochrones become visible.

The fid's from the "Site-Centroids" are carried over to the isochrones in "ftid", so in the isochrone / polygon layer, I was hoping to use:

is_selected(get_feature('Site-Centroids','fid',ftid))

either as a rule in the rule-based symbology, or as filter on the entire feature. In both cases, nothing is ever displayed.

So, is what I'm trying to do possible (show/hide features in one layer based on selections in another) and, if so, is my approach flawed or just buggy?