Average count by pivot subtotal

Hello jbfx and Welcome to Excel Forum.
This proposal employs a helper column, converting the data to a table and Get & Transform/Data Model.
The helper column (C) named Distinct Ct. is populated using: =IF(COUNTIFS(A$2:A2,A2,B$2:B2,B2)=1,1,0)
Note that the helper column may be hidden and/or moved for aesthetic purposes.
After the data in A:C is converted to a table the following Power Query advanced editor is used to produce a connection only to the table and produce a data model:

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The following measure (Average) is added to the data model: =COUNT(Table1[Actor])/SUM(Table1[Distinct Ct.])
In the pivot table that is produced from the data model Distinct Ct. (SUM) and Average are placed in the Values area.
Let us know if you have any questions.