All fine with the above.
But
Once converted to J4 there were pages on my staging site that didn't load. Actually the same pages didn't load on the main site (from which I got the .jpa file for the staging site). The main site is not yet updated to J4 so I assume the error occurred due to one of the latest J3 updates. The server tech support noted:
"The error "500 behavior::caption not found" can be caused by HTML overrides that are not compatible with J4. They are stored at: theme_root/html
After renaming the folder at:
httpdocs\prairiechristmastrees.org\stage2023\templates\j51_kinvara\html
To "html.renamed", the pages seem to be loading now:
https://www.prairiechristmastrees.org/stage2023/about-pctga
https://www.prairiechristmastrees.org/stage2023/photo-gallery
https://www.prairiechristmastrees.org/stage2023/members-only
You can rename it back / review the contents in this folder for further checking if needed.
The Joomla admin has 1 override to check on the homepage notifications which is advised to review as well."