Oh, I forgot you were using SMS Backup & Restore, which is 3rd party. You'd have to discuss this issue with the developer, as you're attempting to do.
I found out what was causing some messages to not appear, after doing a restore in SMS Backup & Restore.
By default, there's an option selected called "skip duplicates".
If I unselect "skip duplicates", then all messages get restored.
Apparently the app must have thought that when I sent consecutive messages in a short period of time, that some of them were duplicates (this only seemed to happen when there were consecutive messages with attachments).
So when I restored to the Xperia, I now saw all messages.
There is one bit of weirdness. When I checked the restore in Google Messages on the Xperia, sometimes it seemed to split group conversations. Like if someone else from the group sent me a message, it would be in one thread.
But if I replied to the group, it would think it was a private reply. I checked the backup XML file itself, and it seemed to save it that way. So that's more of an issue with how the app saved the messages. Overall, this is less of a problem than missing messages.
Maybe something is now in a different thread then it was supposed to be, but at least it's there.
Since I don't have any major blockers now, I'm going to go ahead and get a sim card activated for the Xperia.
I still might try to install Samsung Messages on the Xperia, but if I can't, no big deal. At least I know the messages are all there in Google Messages.
Lastly, I might have done some self inflicted damage. I tried deleting my messages from my Samsung phone and doing a restore. Now some of the group conversations are split, kind of like I described above.
Or sometimes they appear as phone numbers, and not names.
They appeared fine before I tried deleting my messages from the Samsung phone, when I was using Samsung Messages.
One more question:
I'm not a fan of the look on Google Messages where you see the conversation by icons with a circle.
I liked the look of Samsung Messages 1.0 where a conversation was by the full contact name participants.
Is there a good SMS messaging app that will display by their contact name? Or is there a setting in Google Messages where I can get rid of the icon with a circle?