How to sum different cells across multiple sheets based on the criteria on google sheets

[emr]

I have a google sheets spreadsheet with multiple sheets for different clients. On the overview sheet I want to pull data for the specific month i.e March and specific column header i.e Sales from each of the clients sheets if that data appears.

The data isn't in the same cell across each sheet because some clients began in different months.
I have also manually written a list of the clients names that match the tab names so that I can reference them more easily.

The formula I am using at the moment means that I have to manually sum each statement, is there a way to use one formula and reference a range of sheets rather then do each sheet individually on google sheets?

A snippet of my formula at the moment is the below. In reality it is much longer as I have at least 20 sheet names to reference.

=sum(iferror(indirect("'"&$A$29&"'!" &Address(match($A10,Indirect("'"&$A$29&"'!$A:$A"),0),match(C$2,Indirect("'"&$A$29&"'!$A$2:$af$2"),0))),0),iferror(indirect("'"&$A$30&"'!" &Address(match($A10,Indirect("'"&$A$30&"'!$A:$A"),0),match(C$2,Indirect("'"&$A$30&"'!$A$2:$af$2"),0))),0),iferror(indirect("'"&$A$31&"'!" &Address(match($A10,Indirect("'"&$A$31&"'!$A:$A"),0),match(C$2,Indirect("'"&$A$31&"'!$A$2:$af$2"),0))),0),iferror(indirect("'"&$A$32&"'!" &Address(match($A10,Indirect("'"&$A$32&"'!$A:$A"),0),match(C$2,Indirect("'"&$A$32&"'!$A$2:$af$2"),0))),0))

P.S. I have tried sumproduct(sumif(indirect) but didn't manage to get it to work because I was referencing a range and I am not sure if google sheets allows you to?
Any help would be much appreciated and just to be clear I am using google sheets not excel.

Thank you!