Android message merge with iCloud Message history
Solution 1:
For the record, Apple's Move to iOS app does work. In the end, I had to:
- Borrow an old Android phone (i.e. pre Android 9).
- Transfer the messages to that from my Android phone using SMS Backup and Restore.
- Reset the iPhone so the Move Data from Android option was available – only possible when first setting up the phone.
- Use Move to iOS app on the old Android phone to copy messages to iPhone.
- Sign into iCloud on the iPhone to enable Messages in iCloud, which then merged the newly transferred messages with the ones already present in iCloud – I've confirmed that this has worked as the imported messages have synchronized to other devices.
The Move to iOS actually still wouldn't restore to my iPhone XR, so I had to restore to a borrowed older iPhone, then backup that and restore it on the new iPhone… Apple last updated this app in March 2016, predating the iPhone 7, 8/X, XS/XR generations, so hopefully it will get some attention soon to fix these issues, as not everyone will be able to borrow other devices to go through this process!