Creating signal backups for iPhone users is _really_ hard. The only way to do it is to get a backup from the Signal Desktop app, which is also non-trivial because in recent versions of the desktop the decrypt key is also encrypted and store in keychain.
I had to install sqlcipher, find my encrypted key stored locally, find the decrypt key in apple's keychain, decrypt it using Signal's format, etc. This took a lot of trial and error, and reading a lot of existing source (special thanks to https://github.com/bepaald/get_signal_desktop_key_mac but unfortunately it did not work OOTB for me)