I’m confused by what George’s title would be. William was a duke as the grandson to the monarch (and heir ) but got “upgraded” to prince when he was the son of the monarch. Surely when William is monarch, George wouldn’t be a duke?
It’s completely baffling my brain, it’s so complicated
The heir to the English Crown is automatically Duke of Cornwall. The heir to the Scottish Crown is automatically Duke of Rothesay. QE wasn't as she technically could have been shunted down from heir if her father had had a son, ie if he'd remarried after divorce/death and a son was born.
The title of Prince of Wales is a gift of the monarch; it is not automatic. There were several years between Charles becoming heir (and Duke) and him being pronounced Prince of Wales. Previously he was Prince Charles of Edinburgh.
William was always a prince from birth as a male line grandchild of the monarch. Thus, Prince William of Wales, Prince Harry of Wales, Princess Beatrice of York, Princess Eugenie of York, and - though they don't use the titles but can choose to do so - Princess Louise of Edinburgh and Prince James of Edinburgh. This was extended by QE to the main line great-grandchildren hence Prince George of (then) Cambridge, Princess Charlotte of Cambridge and Prince Louis of Cambridge. Now Harry is the child of the monarch, he is The Prince Henry, Duke of Sussex and his children, now grandchildren of the monarch, become Prince Archie of Sussex and Princess Lilibet of Sussex.
A Royal Duke is always a prince first. Their children often use the duchy as a surname in common use; both William and Harry used 'Wales' as a surname in the army and RAF, and people still refer to Beatrice and Eugenie York. However, when the holder is a monarch's grandchild, such as Prince Edward, Duke of Kent, and Prince Richard, Duke of Gloucester who are the grandchildren of George V, their children aren't royal and use their legal surname, ie Windsor in their cases. You may remember Lady Helen Taylor as Lady Helen Windsor and Lady Gabriella Kingston as Lady Gabriella Windsor. Those descended from QE and Prince Philip are Mountbatten-Windsors, which is why Archie and Lilibet used that surname before becoming prince and princess, and why Lady Louise also uses Mountbatten-Windsor as she is not using her royal princess title.
You might find it easier to look at a family tree while reading - those descended from George V include all the Kents and Gloucesters.