The free Corel Paintshop Pro app I got with my camera seems to have tanked on me, and rather than try to reinstall it, I downloaded GIMP. I did some basic color correction in the MS Photos app, then I followed the workflow detailed in the video below:
Here is my photo setup:
Here is the source image:
Here is the masked, straightened and cropped image (6000 x 3000):
Here is the final (draft) collage for social media:
I noticed some masking errors on the image, so I will probably rework the GIMP file and save a better version of the image before posting it on Instagram & Facebook, but I'm pretty happy with the result as a first effort.