Ive used copy bags before but I wouldn't recommend it - even if Qualtex make good copies - the filtration layers aren't as thick and the bags are inclined to clog quickly, thus requiring quicker replacement and thus not cost effective.
Generally I find the GN bags can last up to 2 months to 2.5 months but it depends on what you're picking up. You wanted to go premium, and thus you should stick to premium. Besides you still have that Hoover Jazz cylinder bagless vac so you shouldn't require as many bags for the Miele.