Low sample size, but it took me about 35 oyster items to produce 6 pearls at Artisan 1 Processing and +1 clothes (if it matters). I may have gotten lucky; I suspect actual rate is closer to 10%. At Pro 8 Gathering, giant oyster gathering nodes seemed to give oyster items about half the time, so around 15-20 oyster nodes per pearl. Did not receive any pearls directly from oyster nodes; guessing a 1-5% rate on those.

they need to put more of the giant oysters in the game or make the ones that are there spawn a lot faster than they do. I find it tough to find them and most of the time someone's already beat me to it.

Rotation https://www.youtube.com/watch?v=NTtLXbbSUCQ&t=21s&ab_channel=DennizOutlaw