Open the Galaxy Themes app and click on the person icon. It opens the My Page menu which lists among other item a "Purchased items" selection. Click on that and your previously purchased items are shown and can be downloaded.
You have to be signed into the same Samsung account that was used to purchase the themes, on your current device. If you press and hold a blank area of your homescreen to open up Themes, touch on the profile image in the top corner. There should be an option for Purchased Items. From there you should be able to redownload the previous themes. Please let me know if you are successful.
What about using Samsung Smart switch app on pc to do a backup, then restore on the new device? Wonder if Samsung Smart Switch app backs up themes and wallpapers from Samsung Themes app?