Very weird post, I dont see how a victim with not enough agency to control what apps are on their phone will somehow be able to install a custom os
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?
I am a (very happy) GrapheneOS user, I am certain that I can install a tracking app on it. I can even easily side-load an abusive app that would be banned on the Play Store...
Like I would totally recommend GrapheneOS because it's great, but I don't think it solves the problem of "a domestic abuser can access your phone by making you give access to your phone".