Is there a way to turn off the share panel only for guest / external users? this is a serious GDPDR related problem (data privacy). I need them to be able to access the site but not to access the share panel. In fact, I need them to not be able to even access that panel because searching for users is also a problem.
There is no direct setting to turn off the share panel only for guest/external users, and there is no way to block them search user in share panel.
As a workaround, you could set the sharing permissions for site collection only site owners can share files, folders and the site or people with edit permission to share as shown below: