3D Formula Google Sheets - sum across sheets

I'm trying to script an equivalent to Excel 3D formula into Google sheets.I have a workbook with numerous worksheets and a master sheet that should sum up the data.
In excel I would have used this 3D formula :

=SUM(Sheet1:Sheet4!A2)

Working with Google Sheets for just a couple of months, I have some difficulties going through this.

I tried the following found here on stackExchange:

function sum3D(reference, start, end){  var sheets = SpreadsheetApp.getActiveSpredsheet().getSheets();  var sum = 0;  for(var i = start; i <= end; i++){    sum += sheets[i].getRange(reference).getValue();  }  return sum;}

But this would sum the sheets only defined by their index and would not include future sheets inserted as would Excel do.
So I would like to change this script using only optional argument fromIndex: to sum all values across all sheets from the chosen sheet in my workbook, as ordered by tab position.

That way, I would insert my new sheets after the from Index and would be automatically added to my sum.

Here is what I came with but it's not working and literally driving me mad...

function sum3D(rangeReferenceString,fromIndex){var sheets = SpreadsheetApp.getActiveSpreadsheet().getSheets();var sum = 0;for(var i = fromIndex){    sum += sheets[i].getRange(reference).getValue();  }  return sum;}

Would you be so kind to help me find my way through this ?