Sooo...thanks a bunch for the quick replies. It pointed me in the right direction...I realized that I knew zip-zero-nada about slices and export personas, and found the help video on that (link below), and was able to export 5 sequential snips of my pano to the right dimensions with the right cropping.
Summary of my process...
Pano image is created in camera, not stitched, so it's just a big wide file import into LR
Do main image editing in LR along with other photos. Export to JPG.
Affinity Photo:
Resize image to 1350 height. Crop or expand vertical as needed (I love the Affinity how-to video on expanding!).
Go into Export persona
Create 1080px wide slices across images. I used snapping to get precise on the upper left to begin the selection, estimated width by eye, then used the little transform box to get pixel precise on each width. I didn't worry about adding the +1 to each segment...since folks swipe anyway to see each image in progression, the impact was negligible in my test post.
For my particular pano, I had 5 slices with some real estate left over on the right which I wanted to keep in the image. So I moved all the slices over to the far right, knowing the far left part of the image would be cropped out.
Click the Export Slices button...and done
Then move the photos over to phone and upload to IG.
I was going to use that "upload to IG from your desktop" using the Chrome developer tools trick, but looks like IG wised up to it and removed it from their interface. (If you use Chrome, open Developer Tools, then click on the toggle device toolbar icon on the upper left.)
Thank you!