Are the extra size and weight unwieldy for just sitting around reading emails, watching YouTubes and checking message boards with the tablet in my lap? I use my iPad this way a lot. I’d love a bigger screen sometimes, but will I hate it at other times?
You'll definitely love the larger screen and hate it. The difference between love and hate is a measure of the screen size... the larger the screen the greater the love for it when it "works", the greater the hate when it doesn't. The same is true in the opposite direction... less than a certain size, that difference between love and hate grows too.
With the 12.9 Pro, you need more space to do anything (duh). For example: If you're sitting on a couch using it, it'll be fine... but when you need to put down to get up and get a cup of coffee, you need more free space on your coffee or side table to rest the iPad safely.
Things become more challenging if you're using a keyboard case. It was after a few attempts of using the 12.9 Pro on flights that I decided the limitations of the larger screen outweighed the benefits. (yeah, I know that I'm the oddball that finds the 12.9 too large to comfortably use on planes... everyone else is able to do so without issues)
I went back to the 9.7" form-factor for two years after that, and then down to the Mini 5. Love it. Yes, there are times where I wish it was a bit larger, but it is the reverse situation of what I experienced with the 12.9 Pro... the benefits of the smaller screen outweighed the limitations.
In the end, you won't really and truly know unless you try it for yourself.