I do have the 11 Pro Max to compare the new 12.9" iPad Pro with. Since you are a screen enthusiast, you will certainly notice a difference between those two difference given that the former is OLED. I, too, love the rich deep blacks & contrast of OLED. It's a game changer, frankly. (I just updated my TV to OLED too and WOW!)
A few of my notes:
-One thing that makes a difference is whether you will be watching content in "full screen"/"edge to edge" mode or if you will be preserving the aspect ratio of the content. Obviously, 12.9 will be much bigger than 11 if you are watching in that zoomed in mode.
-The 12.9 DOES have some chunky black bars when watching content with preserved aspect ratio. The height is the iPad pro's limiting factor, whereas with the 11 Pro Max for example, the black bars actually appear left/right of the content, indicating the width is the limiting factor.
-12.9 will be better than the 11 for multimedia (in my opinion), regardless of whether you zoom in or not. Even with thicker black bars than the 11, it still results in a larger content window area.
-No surprise, but you are absolutely going to see a difference between the display and bezel on the iPad Pro, whereas with the 11 Pro Max, that etch is stunningly seamless with OLED.
Personally, I do recommend that you consider trying the 12.9" model. If there is any doubt, try to view in a store before your purchase if possible. Otherwise you can always order to your door and return if necessary.
I think the short of it is: iPhone Pros have a better display, but iPad Pros have a beautiful display too given its much larger form factor. 12.9 for multimedia/full laptop feel, 11 for portability. 12.9 is still very portable too keep in mind, but less so. Both 11 and 12.9, while not OLED, have more than adequate contrast & brightness levels - still a LCD stunning display to be sure.