Did you miss the point, Eric, that 800 mm is much closer to 1200 mm than what the Canon G5X can offer?
I've been seriously wondering if I should splash out on a Canon R5 in order to get the benefits of that lightweight, inexpensive, 800 mm F11 lens. In APSC mode the 800mm lens becomes the equivalent of a 1280 mm lens on an 18mp full-frame sensor.
Of course the EF 1200 mm lens has a wider maximum aperture at F5.6, which is advantageous in poor lighting, or when a very fast shutter speed is required. However, at long focal lengths I find the DOF ridiculously shallow at wide apertures. If I'm photographing a distant bird, I don't want to see just its eye and part of its head in focus, and the rest of its glorious feathers a total blur. F11 or F16 is preferable.