I was on the fence about this very dilemma but opposite of what you have. I have the 12.9 and a few 9.7 and also a mini but i was stuck on which to get - let me tell you that with my traveling and in-car use in the driver’s seat, I settled for the 11 inch cellular iPad Pro OVER the 12.9. The 12.9 is just too big even with the reduced bezel (size being extremely close to an 8.5x11” sheet of paper) - the screen is fantastic for “work” but the size is prohibitive everywhere i am.
The mult-tasking isn’t bad at all on the 11” in fact I find the 11 the PERFECT SIZE. 10.5 would be just a bit too small for me, but remember to treat your ipad as an IPAD not a laptop and you won’t have to buy a larger one. I also have a few MBP 13/15/older 17” and I just use those if I’m desperate for a real computer otherwise with iPadOS now I’m 99% back on ipad only.
Multitasking is really not remotely unusable on an 11inch pro model! Pair it up with a portable keyboard and there is no competition from other manufacturers. IPad is king of tablets.