There really isn't any benefit to staying on iOS 12, save for a few bugs and incompatibilities with specific apps (until those apps get updated).
I've already updated our two iPad Air 2s to iPadOS 13.
Sometimes if it ain’t broke don’t fix it . . I walked into an Apple store with my wife yesterday and showed her the new Air, new standard iPad 10.2 and the iPP 11. Told her I’d treat her to a new iPad and asked her which one she wanted to buy to upgrade her Air 2.
She looked at me and said “Why would I wan’t a new iPad, my old one works fine.” She took one look at the 11 and declared that she wouldn’t want one despite the pencil mount looking “cool”. The folio design was a non-start for her, she wants a standard smart cover with nothing on the back. We ended up walking out empty handed.
She’s still on 12 as well and has been ignoring the upgrade prompt.
I had hoped to at least figure out if a 10.5 was an option for her because the Air 2 is smaller and she likes to tuck it into her bags and has commented about regular ipads being too big.