Air 2 owner chiming in, several in my company with either 9.3.5 or 11.4.1. I still have an iPad 3 on 9.3.5. They're all LTE via VZW. The Air 2 units on iOS 9 are used only for internal company usage with side-loaded apps, I'm going to update all of the server software they're connecting to later this year as a course of action I'd planned last year.
What I can offer is that I agree that iOS 10 and 11 pretty much crippled the Air 2 to a degree. And, I can offer that I'm pretty impressed with iOS 12 on my personal unit - all of my needed apps work, networking works and seems stable so far, and the new OS works as smoothly/speedy as 9.3.5 and it's a huge improvement over iOS 11. At this point the only bit I don't like are the new navigation swiping maneuvers - I'm not getting the hang of them yet. I will be updating the iOS 9 units when 12.1 comes out and after I get to test it - unless Apple introduces some new buggy behaviors as they've done in the past...
I'd offer last that you hold off on installing one of the Beta options, especially if you have a cellular Air 2 - there's still a bit of wonkiness in the baseband and some of Apple's promised features haven't been introduced yet. But, all of my networking apps work, my navigation apps work, my office apps work since Beta 1 - I can't say that from my past experience with any of Apple's previous Betas.