If the backup restore is a merge rather than a nuke and pave it might make sense to do whatever Molly[1][2] does to allow it to be installed alongside Signal, so it's install this, import from WhatsApp, then backup/restore without having to do the uninstall/reinstall dance.

[1] https://github.com/mollyim/mollyim-android

[2] https://news.ycombinator.com/item?id=25743852