Hey Dylan,
I hope you're well. From what I understand about setting the rider sag, 30mm is pretty much a standard number used for the spring to feel not too hard and not too soft, but it's all personal preference. It might be 25mm for a rider who likes it a bit harder for the track. The bike's preload settings to get the rider sag to 30mm will be different for each person because the weight of the rider+gear. If the rider sag is less than 30mm, then the spring will feel harder, and if more than 30mm, it will feel softer. So if your bike is set up for a 60kg racer and the rider sag is 30mm, when someone 100kg sits on it, I'd imagine the rider sag would increase quite a bit to say 45mm (just a guess) because of the extra weight. So the preload would have to be increased to get the rider sag back to 30mm for the 100kg rider. If the preload is at its maximum setting, but the suspension is still too soft, you would need a harder spring to get the desired 30mm. Did I get it right and does that make sense?
Once the preload is adjusted properly for your weight to get the standard rider sag of 30mm or thereabouts, you can play with the other settings like rebound and compression. I hope you have fun with it