Hello cotsy,
I'm sure many users have come across this same issue. I certainly have. When I do, I have to admit I "go off-shore" & use graphics software to complete the task.
In the combined screenshots below, you can see that the edge selected using the Intelligent Selection brush is unable generate a truly straight edge, unless it's graphically generated rather than a photo.
My workaround is (using graphics software) to
draw (using line tools) a crisp edge on the object then use transparency tools to remove the background. I
was going to attach the .png generated in PhotoDirector & the other generated in graphics software, but wasn't able to. They're uploaded to Google Drive instead.
PhD .png file
Graphics app .png file
PhD does some amazing things but, I'm afraid, this particular feature doesn't meet all requirements.
PIX
PIX YouTube channel