Replacing ActiveSheet... with myActiveSheet

I have several macros and functions that are using ActiveSheet.... and want to replace ActiveSheet... with myActiveSheet. But I always get either compile- or runtime errors.

Sub Main()

Dim myActiveSheet As Worksheet
Dim UsedRangeRows As Long

myActiveSheet = createTestSheet("Basis Depot") 
UsedRangeRows = myActiveSheet.UsedRange.Rows.Count 

...

End Sub

Public Function createTestSheet(Optional myTestSheet As String) As Worksheet

If sheetExists("myTestSheet") Then
          Application.DisplayAlerts = False
          Sheets("myTestSheet").Delete
          Application.DisplayAlerts = True
          End If

If Not myTestSheet = Empty Then
          Sheets(myTestSheet).Activate
          End If
          
ActiveSheet.Copy After:=ActiveSheet
ActiveSheet.Name = "myTestSheet"
Set createTestSheet = ActiveSheet

End Function

"Basis Depot" is just a sample value for the createTestSheet function which creates a working copy of this and activates the working copy. The createTestSheet function is placed in my .xlam workbook. The createTestSheet function returns the ActiveSheet "Public Function createTestSheet(Optional myTestSheet As String) As Worksheet" which should be stored in the variable myActiveSheet of the calling macro and should be used in the rest of the local macros.