And now the abuser can just say "show me a picture of your ballot" since we all take our phones into the booth. And the abuser might stand in line and watch to see if you get a second ballot. And you could quickly use photo editing software to fake it, but the abuser might run analysis...
Photos of ballots are a criminal offence in the UK, which obviously is imperfect in an abusive relationship, but does help set a general norm that the vote is secret.
Where is the line of accepted risk?