Tuner will be the best thing to get next. I'm in the same phase you are I need to get a tuner then do my 8.8 swap with some new higher ratio gears.
Then after that you'll be getting into the expensive stuff lol
You'll also eventually want some long tube headers and lower it a well. Long tube headers will get you some better flow an let the engine breath even more. Then after that you can get a bigger cam, frot and rear sway bars, and the bar that goes over the engine (idk why I can't think of it ATM!!!)
But yes a tuner should be your next purchase. Everyone says that's what they see the most gains from immediately out of everything when just starting to modify your stang.
1999 V6 Mustang Dark Satin Green , SR CAI, BBK 65mm Throttle Body, Screaming Demon Coil Pack, SR Lowering Springs, Pioneer Stereo, 2 12' inch subs and 1600watt amp, Green Calipers,