How to make your formula jump rows

Patrick2788 

OK, I'm impressed!  Sorting through the code does take a bit of time and effort.  I came up with some alternatives, not necessarily improvements, that exploit the fact that there are only two data columns to be returned.  The first uses MAP for each data column

=LET(
    month_name, MONTH(DemoTbl[Date]),
    fn,         LAMBDA(v, Thunk(PERCENTILE.INC(v, {0.2, 0.8}))),
    agg,        GROUPBY(month_name, DemoTbl[Val], fn, , 0),
    i,          TAKE(agg, , 1),
    thunks,     TAKE(agg, , -1),
    header,     {"Month", "20%", "80%"},
    pcntl₁,     MAP(thunks, LAMBDA(ϑ, INDEX(ϑ(), 1))),
    pcntl₂,     MAP(thunks, LAMBDA(ϑ, INDEX(ϑ(), 2))),
    result,     HSTACK(i, pcntl₁, pcntl₂),
    VSTACK(header, result)
)

The second goes back to the calculation of percentiles and calculates them separately so that thunking is avoided.

=LET(
    month_name, HSTACK(MONTH(DemoTbl[Date]), TEXT(DemoTbl[Date], "mmm")),
    fn₁,        LAMBDA(v, PERCENTILE.INC(v, 0.2)),
    fn₂,        LAMBDA(v, PERCENTILE.INC(v, 0.8)),
    header,     {"Seq", "Month", "20%", "80%"},
    agg,        GROUPBY(month_name, DemoTbl[Val], HSTACK(fn₁, fn₂), , 0),
    result,     VSTACK(header, DROP(agg, 1)),
    result
)