I think perhaps Charles may have tossed around using George as his regnal name when he was younger but as time passed and he's been blessed to have his mother still at 90 going strong, he may have come to believe that as far as history goes, the bulk of his contribution to Crown and country will be defined by his work as The Prince of Wales than his reign as King.
Knowing that perhaps his reign will be a relatively short one compared to his mother, I think he will prefer to use his given name Charles rather than define his monarchy by opting to be King George VII and leave that regnal name for his grandson to use in due time should he choose.
“When I was 5 years old, my mother always told me that happiness was the key to life. When I went to school, they asked me what I wanted to be when I grew up. I wrote down ‘happy’. They told me I didn’t understand the assignment, and I told them they didn’t understand life.”
― John Lennon