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P900 is only 357mm. The rest is marketing based wordplay. At its highest zoom setting, it's f/6.5 which is very slow.
Max. aperture at 300mm "equivalent" is f/5. I struggle to make f/4 and even f/2.8 work with wildlife a lot of the time, even on bright days. Lighting is a bitch. I wouldn't even attempt using this thing. Pic related is the image quality at max zoom. Absolute trash. Even stacking for astro, you'd end up with garbage quality.
Friendly reminder that 1mm to 35mm is 35X zoom, so don't fall for that nonsense either.