Line shouldn’t be described in rating but in diameter as a lot of lines of the same weight rating have different diameters. Most reels that say a line fits x amount of 30lb generally mean 0.55 diameter line. These days you can get 30lb+ line in 0.45 diameter with most 30lb being 0.50. I wouldn’t go below 0.45 diameter line for fishing off beaches such as birdlings flat. For your shockleader it shouldn’t really be over 9 meters with about two lengths of your rod and a few wraps on the reel being ideal. If it’s a cheap reel make sure it’s as full with line as you can get it without being over full, if it’s half empty this will cause a lot of resistance when you cast. Since you’re only using a cheap rod you can’t expect to much. The rod is going to work better at one end of the sinker rating than the other. Best way is to find somewhere you can cast the rod with a 4oz,5oz and 6oz sinker and measure your best distance. Then depending on the size of your baits you may need to use a sinker size down. Using a pully rig that clips down to your sinker will improve your distance a lot. You can make these easily yourself or buy a pre made one from the shops. Casting a two hook ledger with big baits and a light 4 or 5oz sinker is going to helicopter around and not go anywhere. If you’re casting on a field or down a beach where you can measure your casts you can also try different casting styles and try different things to see what works best.
But it’s true you don’t need to cast far to catch fish, I’ve seen them being caught by people casting into the breakers at beaches like south rakaia and wakanui. The problem is that if it’s really rough your sinker isn’t going to stick and your baits are gonna end up back on the beach. Investing in a possibly longer quality rod would instantly get you more distance.