Thanks! I exported hirez JPGs out of Lightroom and then stitched them using Microsoft ICE. Scary name I know, but it stands for Image Composite Editor. It is now discontinued, but I still have it and use it. I could probably have done the stitching within LrC.
It's good to hear that ICE works, it's been around for a while. I don't use LR or PS (not found of subscription plans) so I'll give ICE a shot. Thanks for the idea.