OK, to make this easy;
1) Make a BLANK form. This is called an "Unbound" form.
2) Add Textbox control for your StartDate
3) In your QUERY, go down to the criteria for that you use StartDate in and use the form's <= StartDate for your criteria. Use this LINK
for help if you are not comfortable using form controls in queries.
4) Add a textbox on your form to hold the [Client ID]
5) For each record, where you currently have
strRptFilter = "[Client ID] = " & rst![Client ID]
you will replace it with
Me.YourTextFieldForTheClientID = [rst![Client ID]
6) You will have the same type of code as 3 to set your querie's [Client ID] = your forms's [Client ID]
This should give you what you are looking for; transactions on or after your startdate and matching the client ID for each of the reports you generate.