I recently moved a site from non https to https. All pages have finally been indexed and I don't see any mobile usability errors. However, the number of valid pages shown in the search console mobile usability report is about half of the total pages currently indexed. Any idea why these pages would be missing from the list of valid mobile friendly pages?
There could be many reasons, and each missing page could have a different cause. For my website, several of my missing "pages" are only images.
I would suggest you export your list of mobile pages and then the pages that are valid from the Coverage section. You may have few sections to export under Coverage:Details. Then using VLOOKUP() in Excel compare the URLs from your Valid and Mobile to determine what's missing in your mobile coverage. Then you will at least know what files/images are missing from mobile and should be able to look more closely at possible issues.