As I said before, it's just my opinion. I don't see Apple going two years between releases. I also don't see them introducing the A10X at the same time as the A11. What really has me wondering about a late March release is, I can't find new 12.9 pros for sale anywhere. Amazon is out of stock, Best Buy is out of stock, B&H is out of stock, MacMall is out of stock, Staples is out of stock, and Walmart is out of stock. The places that do have them are either third party sellers or refurbished units. If I wanted to buy a 12.9 cellular iPad today, I would be hard pressed to find it. To me, that kind of points to a sooner rather than later release date.
So in my opinion, we will see an end of March release.
On edit: everytime I see your username, I can't help but think, "emissary". Lol