Finding threadfins only 50mtrs away, must say you are really lucky ! Whenever I hit those waters, these guys tease me from far at the horizon. Anyway for you, try to cast the halco twisty jigs past the spot where they are feeding i.e. the jigs should ideally land a few mtrs ahead of the area where the mullets are flying. As soon as the jig hits the water, start reeling in, such that the jig skims on top of the water creating a good trail as it comes in; normally these feeing threadfins then hit these jigs. Having said that, I have caught them while doing the normal shore jigging method - where you let the jig sink after it hits the water and retrieve it keeping it under the water itself, but this works mostly when the feeding/action is minimal; you know that these guys are there, but they don't come up.
If you notice occasional feeding on the surface, I suggest you try the "surface jigging" method described above only at those feeing frenzies, i.e. cast the lure only at the right moment you see the feeding. If you observe no feeding at all, try the "underwater jigging" ! Instead of the halco jigs, you can try other jigs, stick baits etc because you never know what might work at different times.
For the techniques described above, actually your gear ratio is good. A gear ratio in simple terms mean how many times the spool rotates when you finish a single crank of the reel handle or just how fast you can crank the line back onto the spool Eg, 5.2:1 means for every full crank of the reel handle, the spool rotates 5.2 times reeling the line in. Faster ratios are normally found on smaller size reels like 5 or below, slower ratios on larger reels and as a general thumb of rule, slower ratios are good for spinning using lures etc and faster ratios for jigging, trolling and similar techniques. Faster/smaller gives more speed where as slower/larger gives more power per crank.
To place the jig/lure at the desired distance at the desired spot is a key factor, to achieve it, use a very balanced system, i.e. match the lure weight with what is specified on the rod you are using. A 9-10ft med action rod, 20-30lb braid with 1mtr leader, 2-3 oz jig/lure would be a good beginner setup I reckon. Hit the beach and practice the casting as much as you can to fine tune the distance/placement.
Good luck !