One additional thing to think about is the possibility of a 4x5 with back-cross movement. (L or R Shift or rise/fall depending on which way round the back is attached )
I have an Ebony that allows me to use 2 4x5's to make a 4x10 - developed together and scanned together, no (or very little) adjustment was needed to levels and the like and in Photoshop, it was just copy/paste - no "stitching" needed. Just a reasonably powerful computer and lots of RAM - aligning one 600MB layer over another is not snappy
I understand that you have issues with water and tree movement etc in panos so this might not fly for you. However, for me a little careful freehand selection at the overlap can make an invisible seam and totally conceal where you have selected one image in preference over the other because of a motion issue.
For me the appeal of Readyloads was major. I have tended to do 4x10
rather than 5x8 with this approach, simply because my printer is only 24" wide and I can't reap the benefit of the 5x8 over a cropped 4x5.
The back-cross option is one I use as frequently as I suspect you'd use 5x7 after comparing 5x7 filmhoders and ReadyLoads for a while