I set up the highlighted content web part showing all the communication sites. So far so good.
However, if I change the SP group #Write permission to READ in the site pages library, the highlighted web part won't show that communication site anymore. I am not sure if this is a bug or intended? Users still have access to the site and they can read the content and upload files into the doc libraries.
The custom permission from write to read on the "communication site page" is needed because the users aren't allowed to edit the communication sites but they are still allowed to create news which are site pages as well. Therefore, I applied unique permissions on that element only to achieve this.