Regretting buying this 14mm now that I got it

There are no other issues created by the user. You did a perfect job. If you want straighter out of the camera you would need to be further away with a longer focal length and ideally raised up to midway in height.

However, the user could have easily have created issues if the camera was slightly rotated. In that case the correction would be two steps, the other involving rotation. With wide angle work those gridlines in the viewfinder are critical as is centering... Since you had the orientation spot on the correction was simple