To the OP it makes clear sense no doubt.  In this case the OP says they have part of the solution but only offer pictures of the problem.  Maybe we...

It seems dfp(start_dt) should've been dfp(dt) in two places.   This is a really slow way to debug 🙂

with
dt_cte(dt) as (
    select dateadd(month, fn.n, dfp.start_dt)
...

dfp(start_dt) is (or was intended to be) the alias (with column label) for the object defined by the VALUES table value constructor.  It appears there some were missing parentheses in...

By "regroup"  does it mean "round and expand"?  Otherwise, how to handle the decimal precision?  To create 1 foot increments based on rounded ranges you could use a tally...

ktflash, afaik it's almost a correct "quick fix".  TBL1 is a recursive CTE which generates a sequence of months (1 to 12).  Presumably this date range is to be adjusted...

Afaik this helps going in the other direction, from @thedate to iso week.  What about iso week, iso month, iso year to calendar date?

Thanks.  I'll look at it in a bit more detail later but my first impression is that the following line of code in the CROSSAPPLY is incorrect.

Aus dem Paradies, das Cantor uns geschaffen, soll uns niemand vertreiben können