After a couple hours using my 12.9 M1 iPad Pro, lemme address the shadow stuff. It’s really not a problem to me… and I’m someone who tends to obsess over display quality, uniformity, etc. (I exchanged three or four 12.9-inch 2018 iPad Pros over this stuff.)
It’s not really a “line” around the border so much as a very thin, barely noticeable gradient. It’s there if you specifically hunt for it in settings, on a white Safari page, etc. But as was mentioned earlier, the uniformity of it — that it goes all the way around the screen — basically makes your eyes completely ignore it.
Also, nothing about this display seems lower beneath the glass than my 2018 unit did. I’m not noticing any increased gap, so I’m not sure where that perspective is coming from.
Enjoy the iPad, people. It’s an amazing device.