We go to Cornwall most years and have just got back from this year's holiday.
We deliberately avoided popular spots like St Ives, Newquay, The Eden Project etc which are generally always busy.
The places like Penzance, Hayle, Cape Cornwall etc which are usually quiet were just as quiet as usual and just as beautiful.
We had a lovely time. You have to rebook most things like The Minack, Flambards and Jubilee Lido but that's fairly normal - although a bit of a pain when the weather has been so variable.
What we did find is a lot of restaurants seem to be operating at a very reduced capacity though. I think due to staff shortages.
There were several times where we'd enquire at a pub at around 5.30pm to see if we could have dinner and be told there was nothing available at all or not for several hours. Generally the pub would be almost empty at the time.
At one pub, with literally 3 customers in it and none of them eating, where we were told we would have a 3 hour wait for food. The waitress did say it was to do with how many people she could serve rather than capacity. She then promptly went back to chatting with the bar staff who were standing around with no one to serve either .... go figure.
We did however find a few good places that didn't need booking, had enough empty tables and were happy and able to serve nice food to all the customers that came in, so we were happy to stick to those places.
As long as you use common sense and are flexible with your plans you'll have a great time.