Howdy and welcome to site - lots of info here, so enjoy looking through.
I'm not sure if you can get sand worms from Sellicks (maybe wrong), but there a lot rocks/pebbles along the beach (more noticeable when sand has been washed away after some rough weather). Plus vehicles harden it all up too.
Maybe Silver Sands bch (but may have hidden rocks/pebbles also).
Aldinga bch there would be a chance.
Can get them at Port Willunga (north end), Maslins bch (definitely north end ) and Moana bch (south end)
Good luck and I'm impressed you're trying for them - hard buggers to