Formula to combine subtotals for one type

No problem, the report type you are currently using is called a table report. That is just a list of records.

Make a brand new report but this time choose report type  Summary. It will let you configure which field(s) to total and it will let you group by the Stage.

The report will allow users to drill down into the detail that makes up the totals. If you'd like you can show a count of the number of underlying records as part of the summary report.