Simplify a complex formula

If you want to shorten by getting rid of the long "array constant" with the sheet names then one way is to list the sheet names somewhere on your sheet, e.g. in Z2:Z13, and then name that range Sheetlist and use this formula

=SUMPRODUCT(SUMIF(INDIRECT("'"&Sheetlist&"'!B:B"),$B7,INDIRECT("'"&Sheetlist&"'!"& ADDRESS(1,COLUMN())&":"&ADDRESS(20000,COLUMN()))))

alternatively you could use a formula to generate the month names, e.g.

=SUM(SUMIF(INDIRECT(TEXT({1,2,3,4,5,6,7,8,9,10,11,12}*29,"'mmmm'!")&"B:B"),$B7,INDIRECT(TEXT({1,2,3,4,5,6,7,8,9,10,11,12}*29,"'mmmm'!")&ADDRESS(1,COLUMN())&":"&ADDRESS(20000,COLUMN()))))

....and using the first version with a simplification of the ADDRESS part....

=SUMPRODUCT(SUMIF(INDIRECT("'"&Sheetlist&"'!B:B"),$B7,INDIRECT("'"&Sheetlist&"'!"& CELL("address",INDEX(1:20000,0,COLUMN())))))