Don’t know about the 12.9, but iOS 12.2 is widely reported to have solved glass screen protector touch sensitivity issues. I experienced a complete resolution myself.
I’ve been using Zagg protectors on my iOS devices for many years.
No problems and very happy.
There’s a kiosk in the mall right outside the Apple store. I let them handle putting them on and it’s always perfectly applied.
Both my wife and I use them.
My 2 cents...
I just picked up the Zagg Glass Plus for my new 12.9 IPP. I had BB do the install saved me a ton of headacks for a small price. It came out so perfect that I tipped the BB guy. I just want to chime in and say that Zagg is outsanding and the best interation of their screen protector yet. I've been using Zagg for many years. Those previous Glass protectors we're as smooth as the iPad Glass naked. But this Glass + has some resistance causing the Ipencile to feel like I'm writing on paper. I'm really happy with it and the clarity is great. This is not the case with other "paper like" screen protectors. The truth is I didn't expect this at all. nZagg is great! But their keyboard “Zagg Slimcase Go” sucks!.