Sum YTD and Quarters using Index and Match and Sumifs between months across columns

Hi, I'm working on a new spreadsheet to show YTD and Quarterly activity.
My data set is organised with the months being horizontally across the columns and the activities vertically down the rows.
Spreadsheet uploaded with this post (with dummy data).
My data set will have new rows and columns added each month.
I have tried Sumproduct but the range needs to be specified and is not dynamic allowing new rows or columns each month.
The attached spreadsheet is using Index and Match and Sumifs.
I would like to continue to use these if possible to calculate YTD and Quarters QTD (see cells highlighted in red) with criteria in droplists for the start month and end month (see cells A1 and A2).
There is multiple criteria so the individual activities can be itemized per row.

Please help... my brain is about to explode!
I've spent hours looking through the internet and Youtube and just can't figure this one out by myself.

My current formula for MTD is as follows:
=SUMIFS(INDEX('YTD stats 2019'!$J:$ZZ,0,MATCH($A$2,'YTD stats 2019'!$J$2:$ZZ$2,0)),'YTD stats 2019'!$B:$B,A7,'YTD stats 2019'!$C:$C,B7,'YTD stats 2019'!$D:$D,C7,'YTD stats 2019'!$I:$I,$A$3)