I agree also, best to send it back if you're not happy with it.
I always buy used and also I check them against the data on this site:
https://www.imaging-resource.com/ after buying to see if there's anything that doesn't match with their findings - if it doesn't match reasonably closely, it goes back.
Their sharpness maps are very easy to digest, and one can easily see if there are any softness anomalies at the corners etc., at various f-stops and focal lengths, Not all lenses have been lab tested, but most have though, and if they encounter any weirdness, they sometimes test several copies.
I took the liberty of looking up your lens, and it seems there is a bit of unevenness corner to corner
(the top right side does look a little softer - worse on FF), but perhaps not to the extent you describe, and certainly not to the extent that show in your images.
The lens they tested seemed to perform quite well on crop sensor cameras, which would confirm that you did indeed make the right decision, by sending it back.