The diaries of the 'Magnificent 7' will be in some disarray, as they work out who can cover the Queen's engagements.
Prince Charles might perhaps have been able to travel over the summer (but wasn't there talk of the Sussexes coming to Balmoral? It came to naught, but might have influence planning at the time)
Formally, only Counsellors of State can represent the Queen. It might be time to look again at the list. It's the Consort, then by line of succession of those aged over 21
So Currently it's Charles, William, Harry and Andrew. I think Harry could be suspended from the position as he is not domiciled in UK, and Andrew really needs to be removed - but I don't think there's a mechanism for that. Maybe he could consent to standing down?
That leaves Charles and William, and the next to add would be Princesses Beatrice and Eugenie. There might be a case to add Camilla early, as she is expected to become Consort
Though of course any of them can cover wider duties (the charitable and glad-handing stuff)