With the benefit of hindsight, the one thing that was cancelled was her participation in the State visit to Singapore in November 2023, at the time ascribed to George's public exams. That may have been the exact truth, because he would have been sitting entrance exams at the time, or it may have been a contributing factor which, combined with whatever-it-is medically, meant it added up to a decision that remaining in UK was the right thing to do. Also, an unexpected window in her diary might have been an opportunity for medical consultations and tests.
If (big if) that was the case, it was handled really well as there was zero health speculation, and the announcement just after the operation came out of the blue.
It's also possible that before the operation started, there were a range of options about what would be done depending on what was found (and consent given to any/all of them).
And I think the cancer findings were genuinely unexpected, as William went straight there, even though his absence from a family memorial service was bound to be noted and provoke speculation about what on earth was so important it couldn't wait a couple of hours.