Would someone who has been using the P900 for a while be willing to suggest the most important settings to save to U for those of us anxiously looking forward to their much awaited back-orders to arrive? o

o
Crazy, thought of a couple of other things...
Using "Active D-lighting" slows buffering, so best to use "low."
The Program mode has something called "flexible P." You engage it by turning the dial above the thumb rest while in P mode, which then allows you to prioritize either shutter speed or aperture by using that dial and the ring on the back of the camera. The default setting assigns shutter speed to the upper dial, and since I find it easier to manipulate than the ring (and I'm often wanting to increase SS) I leave it that way.
On the SX50 I used Auto ISO, but on this camera I use "Fixed Range Auto," leaving the camera set to 100 to 400 for use with P and S modes, and setting it to 100 to 800 (with flexible P and D-lighting) for the custom U mode.
I set minimum shutter speed to 1/125 for general use and 1/60 for U. The idea is to get as close to optimal settings for IQ as possible, but have a back-up setting for more challenging situations readily available. For the same reason I leave the AF area set to spot, but have it set to normal for U. You might prefer to reverse that, since many people find the normal AF square to be more accurate for birds. I'm still undecided so I've also set the FN button to AF area, to be able to change the size of the square quickly, but stay in the same shooting mode.
Many people also like the "continuous high" (burst) setting, but I prefer "continuous low," which doesn't slow buffering. If you leave the "Scene" mode set to Birdwatching or Sports though, you can access continuous high fairly quickly with a turn of the dial.
The flash only appears to work when "single shot" is used, so that's another setting I've saved to U. Basically, if I want to use the flash, D-lighting, digital zoom or allow higher ISO and slower shutter speeds, I just switch to U.