PowerBI dax measure

I have dax problem, can be a simple one for experienced power bi person.
Basically, I have the ReportingMonth on slicer which is date column. and I have following measures.

Count Open this Month = CALCULATE(COUNT(TestData[ID]))

Count (Last Month) = CALCULATE([Count Open this Month],DATEADD('TestData'[ReportingMonth], -1,MONTH))

I have these values in KPI visual.

Now I need to replace ReportingMonth in slicer to below column.
Current/Snapshot Period =
IF(
TestData[ReportingMonth] = CALCULATE(max(TestData[ReportingMonth]), all(TestData)),
"Current Period",
FORMAT(TestData[ReportingMonth], "MMMM YYYY")
)

Which is text column.

Update "Count (Last Month)" measure so it can work when the slicer is changed to "Current/Snapshot Period" instead of reporting Month.

Attached the pbix for reference.