I mean, you don't even need to use page nodes anymore.
1. Create an article forum, not displayed in list, give it a node name like portal
2. Set the indexPageRoute to forums/portal/
3. Create threads or move threads to that forum
That's your "portal".
There is of course more advanced things you can do by adding widgets. Whether that is complete enough depends on whether you have the widgets you want in the core or via an add-on.
You can target the display of widgets so they're exclusively only shown in the sidebar of the article forum you're using as your portal page using widget display conditions.
1. Create an article forum, not displayed in list, give it a node name like portal
2. Set the indexPageRoute to forums/portal/
3. Create threads or move threads to that forum
That's your "portal".
There is of course more advanced things you can do by adding widgets. Whether that is complete enough depends on whether you have the widgets you want in the core or via an add-on.
You can target the display of widgets so they're exclusively only shown in the sidebar of the article forum you're using as your portal page using widget display conditions.