Add Rows to Nested Table | Group | Offset

Dear Experts,
I hope you're all doing well.

1) How do you add each current row's [GL Code] and [Dr./(Cr.)] into each nested table?


2) Here's the desired result:
a) Grouped by [GL Code] then,
b) Offset amounts to [GL Code = 2010] <<< how do you do this?


3) For cleaner M codes, my current Table.AddColumns took 6 steps [Taxes.1], [Taxes.2], [Taxes.3], [Taxes.4], [Taxes.5], [Taxes.6]. Is there a better way?
Sample file is attached.

Thanks in advance,
Ricky