Hey there JoeBrug!!
Thanks for allowing me some time to get our collection to the north side of the house as they are saying 90+ to over 100 for the next week. Last week we only had a couple days in the high 90s and that about did me in trying to keep them cool!
So you want the tree form, you know you will have to keep shaping to keep them
like a tree. Brugmansia suaveolens Frost Pink, I will tell you is a shrub and no matter how you try it wants to be a shrub. Some of the other suaveolens you can
train to be tree form. Just snap off the lower limbs to the first 'Y' which you will need to for blooms.
A great book which so of us call the Brug Bible is called Brugmansia and Datura
Angel's Trumpets and Thorn Apples by Ulrike and Hans-George Preissel. They do the best job in educating one about Brugs and Daturas. You can google my name and find my blog site where I do have more information.
After you get a trunk to 'Y' I recommend one switch to a bloom booster fertilizer as you have the growth now you want them to product buds. A regular 20/20/20 fertilizer is what I recommend weekly to get them to grow! Scream at me if you need more help.
Chicago, one might not get blooms until fall. Happy Growing.....these can be addicting once you smell their evening blooms!