I think you should totally uninstall Whatsapp on the new device before copying the files. When I migrated Whatsapp from an Android tablet to the J2: with the files already copied and Whatsapp already opened a couple of times, it offered the menu “create new account” or “open session” (as secondary device). This isn’t good.
It must be the menu where it only offers 1 possibility, to enter the phone number, which I think should happen on a fresh and it discovers the files. When that happened on my side, it just asked the number then recognized I had a backup and offered to restore it.
After succeeding, I now follow a full backup routine according to this:
I backup both folders to my computer through rsync, now on a hourly basis. If both folders are copied on another Jolla Phone one day, I expect that it will not even know it moved to another phone.
/home/.appsupport/instance/defaultuser/data/data/com.whatsapp/(the app storage, only accessible on a rooted phone)/home/defaultuser/android_storage/Android/data/com.whatsapp/(the usual one you knew about)