I've done a bit of research. There are a few articles out there quoting "royal observers" as saying Charles will be/is a caretaker monarch, but none quoting him as saying that. Anyone can be a royal observer, including most of us on these threads.
Charles has essentially been putting his own stamp on the monarchy, in his role as Prince of Wales, since he was in his 20s. His reign is in many ways a continuation of that. I don't think he sees himself as a caretaker for William any more than any monarch likely wants to preserve the institution for his descendants.
And by the way, Happy New Year, everyone! I'm busy making the traditional Southern US New Year's good luck feast: black-eyed peas, rice, ham, greens (in this case creamed spinach), and cornbread.