A watercolor struggle continues

Cathedral Basilica--Santa Fe 4 x 6

My brother has commissioned a 22 by 30 watercolor for his adobe style home in Lubbock of one of our favorite places, El Catedral Basilica de San Francisco de Asis in Santa Fe. Of course he wants a night time snow scene.  
He asked for this three years ago, and gigs me about it, as he should.
 I've started multiple times, mostly unfinished, kept them all, torn some apart, collected magazine photographs, photographed it many times myself, collected other artists' renditions...I painted a smaller one 15 years ago as a gift to friends in Santa Fe, but it was daytime, and ...well. 
Since then, I've started more times, quit dissatisfied because they lacked life, or were just architectural representations, not originals. And I've got lots of quick studies, and Christmas cards I've painted, all from memory now, very little sketching.
The problem with a bigger piece, at least for me, is that the journalist in me wants it to be proportionately correct at least, and then go beyond. So here today, thought about as I was driving down the road, is a 4 by 6 quick study--no sketching, only memory and ideas. If I could be as free on the big sheet...alas, type AAA wants to be in control. Not good.
Now I've gone back to the big sheet, trying to mark with pencil the proportions, but still intimidated. Have to work fast, big brush, and let things happen. Soon, I hope.

And, oh, here's the photo in my studio scattered with Cathedral artifacts, photos and failed attempts  I sent to my Bro to con him into thinking I'm working on it. 
Each attempt teaches me things, and opens possibilities and cures problems, but I'm well aware, too much of this will rob spontaneity from the big painting. Time to just do it, I guess. It's only paint and paper. But I work on it a little, then back away, then find another excuse to delay the struggle.