So i didn't collect all 100 stars however im 11 short, should i buy the 11 or just not worry about it

will not having those achievements keep me from the loyalty box when I hit 5years?

Definately not. You could have 0 Achievements and once you hit the 2 year mark you'll still get the box.

It is up to you if you want to buy them But it will not stop you getting the Loyalty Box

Phew, I was getting concerned xD