I actually liked the 12.9" and I have the 11"
Yes it is a little weighty, and a little cumbersome, but you get used to it and adapt.
This was the very same issue when I first jumped to the Max iPhones, I was sure I would never get used to it, but I did, and absolutely love my iPhone 12 Pro max.
The main thing is do not buy both! Because this will really annoy you, because one minute your loving the 12.9, then you pick up the 11" and then you start to love the 11". Just buy what you want and enjoy it, give it a year and if you still aren't happy, upgrade and keep that form factor because that's the one you'll enjoy most.
I'm actually thinking of jumping back on the 12.9 M1 bandwagon, I just loved the screen estate for editing Lumafusion videos and watching films, YouTube etc