I know Cornwall well. If you want to explore St Ives beaches I think you would have the best experience staying right in town for a night or two. There are loads of restaurants, bars and shops and it nice for an evening stroll as well as for hanging out in the day. In St Ives Porthmeor is the surfy beach, Porthgwidden small and cosy tucked in by the ‘island’, Porthminster lovely and calm... very family friendly. Porth means ‘beach’ in Cornish. Harbour Beach is lovely for an explore when tide is out, walking round the boats. They are all in town easy walking distance from each other.
Carbis Bay is one stop up on the train or a nice longer walk Along the coast path. It has a big hotel right on the beach. Food and drink expensive! Perfect for SUP or kayaking.
Next up the coast is Porth Kidney, massive expanse of sand with no facilities. Best way to access it if you have a car is to park in Lelant village and take the path down through the golf course. Best one for getting away from people or for a long beach walk. Never crowded.
Porthcurno is beautiful and lovely for a beach day trip, go EARLY on a sunny summer day to get parking. Actually it would be rammed on a sunny summer day, I’d avoid unless you have the luxury of a sunny day out of season. Also careful swimming there, someone died there yesterday. Well... be careful swimming anywhere! Look for lifeguard covered beaches if going in the water is your thing, cover is patchy so far this year.
I could go on for hours we have many beaches! Different nice things about them all, But some issues too. Some have facilities and easy parking, some get super busy, some you have to walk a bit to get to. Depends what is important, if you want to people watch, swim, park easily, Do water sports, get away from everyone... whatever. You make your choice depending on what experience you want.