The 12.9"iPad Pro is one of the few pricey electronic devices I've found myself buying and then not really using. I had a lot of ambitions for using it for instructional-type drawing but I didn't really get that going. And it's just too big and heavy to use generally. And I say this as someone who really likes its smaller sibling, especially when I'm traveling.
But the big iPad now just sits on a stand in the kitchen where I use it for recipes and the odd video. It's pretty underutilized.
The 12.9” is rather large, I certainly use it less than I anticipated because of it since it’s not particularly friendly to hold for expected periods. When I eventually upgrade I might consider getting the 10.5” model as a result - though I enjoy the larger screen still for certain activities like reading replica newspaper and magazine subscriptions.
I like the 10.5" more for most purposes although, as you say, every now and then the larger screen is useful. If it's just flowing text I'm reading, I actually prefer the Kindle Paperwhite but the iPad is sufficiently more versatile that that's what I almost always bring when traveling.
But the big iPad now just sits on a stand in the kitchen where I use it for recipes and the odd video. It's pretty underutilized.