Regarding corner softness:
I own a Canon SX230HS. I’ve installed CHDK on it. I get truly raw images, without any funny business from the manufacturer. I think most people would be shocked at how much manipulation of these photos is done in this class of camera, especially stretching pixels, before you see a “raw” image.
A lens with this much zoom produces an image circle that is (way!) too small at wide angle. Pre-manipulation, this camera probably produces images that have no data whatsoever in the corners (black) at wide angle, with a fisheye-type circle of light in the middle, that is stretched out to fill the frame to give you a “raw” image. Hence the extremely soft corners at wide angle. There should be a sweet spot somewhere in the zoom range, though, where you can get relatively sharp corners, because the image circle fills the frame without the need for correction.
Travel zooms are what they are. I love them, but they aren’t for pixel peepers.
Started out doing photography at the age of 6 using an uncle's old 1940 kodak brownie box camera. At 15 years of age, I decided to buy my very own 1975 Praktica SLR camera. I now shoot with a Nikon D850. I do unpaid TFP and commercial paid work.