What a lovely pattern - and a super cosy cardi to wear!
Notes on my modifications below.

COLLAR:
119 sts picked up for collar. I picked up all sts, including on top of the button bands, to achieve a wide all-round collar.
I started knitting with gauge needles and increased needle size twice across the length of the collar, to achieve a looser top.
2 purl sets added 2 rows before bind off, for stretchy bind off to go knit-purl in rib (121 sts for bind off)
Used Jenny’s surprisingly stretchy bind off to finish. Another great alternative for a wide collar would be tubular bind off.
The total height of my collar is 19 cm (from sts pick up point).
Note: since I increased the height of my collar, I also added the 6th button - just in proportion to the rest of the buttons. You cannot see the button when the cardi is unbottoned or buttoned, because the collar hides it - but it surely does the work!

SLEEVES - CABLES:
After picking up the stitches from the st holder - started the patt on sleeves from patt row 2 (no crossed cable sts on top of sleeves to keep the shoulder softer)
Only 2 k sts; p on row 3 on the sleeve

SLEEVE DECREASES:
First 10 cm no decreases, then dec every 11 rows 6 times in total
44 sts remain, start cuff and at the first row dec 4 sts
23 ridges of purls on sleeve