Hi Luca,
Thank you for the question.

It is not doable out of the box or in simple ways.

There is a workaround to filter activity based on user id from a specific member type. The method is not efficient and will become a bottleneck for even moderately active site.

Here is how to achieve that
Filter on activity query
fetch all user ids belonging to the specific member type.
Limit query to users belonging to these list.

That seems the only way to achieve it.

Regards
Brajesh