I have been wondering if there is a feature or any 3rd party tool which could help me creating a report to visualize not just the distinct count closed deals of a team but also the average per person linked to that team. (Number of deals / count of team members)
there is currently no option to make calculations like that in the HubSpot report builder.
You could get there with 3rd party tools though. A combination of funnel.io and Google Looker Studio can get you there. Funnel.io has a free plan, so I think this should be possible without any additional cost.
You can add external content to a HubSpot Dashboard, yes. If you create a simple Google Looker Studio Report with the data from funnel.io, you can embed that into the HubSpot report.