I have a Jane in full sun in zone 7a(VA) and flowered really well this past spring. I trim it for now (put it next to a Japanese maple) so it doesnt overtake the JM(Bloodgood) and I want it a bit bushier, but it is a fast grower, at least in these conditions. Bloodgood is also growing fast so kind of trying to keep them about same size so one doesnt shade the other out. They are both 5-6 feet tall.

the first thing i suggest.. is that you not plant it.. until the proper planting season ... see link ...

https://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub

i dont know what if any impact being in the mountains might have on timing .. or any other variable ...

usually .. when you see descriptive.. favoring morning or evening sun ... the point they fail to make.. is get it sun NOT IN THE HEAT OF THE DAY ... this is usually recommended for heavy water use plants .. for example.. hosta love sun ... but they burn if they get too much sun in the heat of the day.. when they cant pump enough water into the plant to offset such ...

but then ... they throw in full sun .. whats that all about.. lol ... i would intuit from all this.. that the further south you go .... perhaps ... you need to worry about the variable..

one trick i have for seeing the future is here:

https://duckduckgo.com/?q=magnolia+x+jane&t=ffcm&iar=images&iax=images&ia=images

ken