If you've got the money to do it - go for it! Either the 12.9 or 10.5 are well worth having and a substantial upgrade from the Air 2. I went from the 9.7 pro to the 10.5 and am very happy with it (although I do look wistfully at the 12.9 when I walk past them in shops - it IS too big for my usage, but wow...). Some would consider that a 'minor' upgrade, but it's made a huge difference to me and I use it so much more than I ever did the old one.
My brother made the switch from the Air 2, that he was perfectly happy with, to the 12.9 and loves it. He's got a really bulky, protective case on it and doesn't care about the size or weight. He can't get over the screen, the camera... IOS 11 is really going to make the 12.9 come into its own. He kept telling me to get the big one but I stuck to the 10.5. I think when I see his in real life next year I'll be very, very jealous.
Air 2 owners were very lucky because it stayed the flagship model for longer than usual and is still perfectly good now, so you may feel you actually don't have to upgrade. But, if you want to, and can afford it, either of them would be great. The 12.9 would be the most noticeable and dramatic upgrade though - you'd really know that you had something new and different...
On the price front, yes, it is as expensive as the phone, but you get so much more screen real estate for your money. I can't afford to upgrade both this year, but think now I've found the joys of the iPad, I might focus on those and not worry so much about having the latest phone. I may well change my mind when they come out if they're amazing, but I'm getting much more value out of my Pro than I am out of my phone nowadays.