I also agree that, if Starmer gets anything like the kind of majority that's been predicted then he'll be secure for the full term.
But, if the majority is smaller than predicted, then I can see things getting interesting.
If he doesn't get an overall majority - which seems highly unlikely - he's toast.
From what I've gathered, there's little love lost between him and Rayner but leader and deputy don't have to get on as long as they have a working relationship. As things stand right now, I think they're too useful to each other for either of them to want to change that. But things can get volatile very quickly so who knows.