With Labour it's got to be someone that appeals to all potential voters. Rayner, I'm not so sure. Maybe if (like I suggested for Sunak) she takes the time to learn, develop, and grow, for a future time.
People are turned off by militant tendencies. See Jeremy Corbyn.
As for Brexit. I personally would benefit from being in the EU (easier to move abroad) but it's unhelpful to dismiss any and all concerns. Like most things in life there were pros and cons.
One of the reasons why we're having a right wing Brexit is because the Labour Party refused to accept the democratic vote.
We didn't/don't have to have a right wing Brexit. There was Labour Leave.
Labour's best bet is to get behind Starmer. At least give him a fair go (which he hasn't had). He does need to make some changes including distancing himself from Blair, quit stoking regional division, and remember disabled people.
But if he and his Party are willing to work on some of their issues there's a reasonable chance.
Otherwise they too will destroy themselves in their own game of thrones.