Hi @FixitFelix,
yes, it’s possible with authenticated proxy and profiles in content filter.
http://docs.nethserver.org/en/v7/web_proxy.html#authenticated-mode
http://docs.nethserver.org/en/v7/content_filter.html#web-content-filter
I saw you already found this detailed howto for AD/authenticated proxy.
In web content filter you may create profiles to map users to filters:
It’s easily installed and tested. You may have problems with some devices/software but they could be excluded.from proxy.