Just thought I would share how I made my Axon m work for me as, out of the box, there are a lot of things that could be done better with their software.
For some odd reason, the stock launcher will not rotate in extended mode. So I tested just about all the major launchers, and settled on Nova, as it maintains the icon grid and layout in both 1 screen mode, and extended mode. Landscape or portrait. The down side of Nova is it only shows half of your home screen in A/B mode, but I use that mode so rarely it didn't matter to me.
Next, and probably obvious to most by now, is go to settings/display and enable "force apps to work with dual screen" and "extended mode for all apps".
Third, I set my display size and font size both to small. Personally, I really want root to set the DPI even lower, as I think it should be with so much screen real estate, but it is what it is. You can change the "smallest width" option in developer options, which seem like an obvious fix. But, as soon as you change screen modes again, or reboot, that setting will reset. If you leave your phone in one mode, though, you can increase that number to suit your needs. I like it to be at 731 in extended mode. But like I said, you have to set it and then leave it in whatever dual screen mode you're inn, or it will reset.
Last, I got "ultimate rotation control" from the app store and set that app to force portrait mode, but you can set it how you like. If you set it to force auto, the stock launcher will rotate in any mode, which is great - though it looks a little funky. I set it to force portrait as in single screen mode, I want portrait. But in extended mode, I want the bezels to be on the sides rather than top and bottom, and forcing portrait mode will force that rotation for you. Why don't I just use force auto? Because I find the rotation sensitivity to be a bit high, and the device rotates more than it needs to. Also, I find that this device lags quite a bit (particularly from screen off to on) when rotation is enabled... and I can't take the lag.
I think that's really it. Unfortunately the performance on the device is never going to be perfect on nougat with a SD821. There is just too little background process control and not enough power and/or RAM. But I still like it enough to use it as my daily driver most of the time. There are still plenty of issues (camera, screen density, etc) but this isn't a review... just trying to help other users out a bit.