I was afraid to make a jump from 10.5 to 12.9 M1. And oh boy no regrets here.
I use it very often as a laptop on magic keyboard. I put it in smart folio case while I use it as a tablet on couch, scrolling through internet or watching content, reading books. Either siting or just putting it on my chest while I ile on bed. I don’t use it to write comments or answer to tweets in tablet mode, for that I definitely need to dock it to MK. But then I get amazing laptop-esque device. But it’s my use case - reading a lot of materials, surfing web, occasional YouTube, trying to learn to draw, using it for note taking.
So answering your questions - no problems with using 12.9 in bed for me.
Of course 11’ inch will be more portable, more comfortable in tablet mode. But 12.9 is just amazing and it’s completely possible to use it as a tablet. Perhaps not that much as 11’ but it’s still doable. Just a matter of getting used to the size.
PS: Nowadays I‘m leaning towards bigger devices. It’s easier to get used to big screen and work around it to streamline experience than buying smaller and be stuck with it. You can’t work around small screen.