Installing Graphene can be done through the browser: https://grapheneos.org/install/web

It's not as easy as it can be (the text is aimed at people familiar with Android flashing) but in practice you need to toggle one setting, reboot holding the volume button, and then click four buttons in your browser in order, with the exact names for settings spelled out in the guide itself.

I don't think wiping an abuser's malware is such a great solution unless you've already managed to get out of the DV situation. Perhaps GrapheneOS is a good idea on a secret second phone?