Honestly, of all of the current justices, I'd bet on Roberts being the next to retire.
He just seems fed up with trying maintain the peace on the court.
I actually wouldn't rule out the possibility of Roberts retiring under a Democratic President. I remember when the left feared the possibility of a Chief Justice Scalia. That wasn't really a realistic scenario. Justice Scalia really did not have the temperament to make a good Chief Justice.
As for Justice Breyer, maybe Bill Clinton needs to talk to him as to the severity of the current crisis in our democracy. A 7-2 conservative majority would be unstoppable.
I agree Roberts hanging it up soon is a very underrated possibility. If it happened within the next 2 years, I guess we would get Chief Justice Kagan? Also worth noting that a Roberts retirement, even under Biden, would probably push jurisprudence to the right. Roberts has been leaning on Kavanaugh (and occasionally even Barrett) with some success. If he's gone and the new CJ is a liberal who is going to dissent anyway, it will be Thomas and Alito leaning on Kav going forward.