Austin I went through the same STP as you and jump 30 landed on the windsock from target fixation from 300 ft... I saw I was gonna land on the windsock but was afraid to do a "low turn" since so many people get hurt and I'm still new to the sport. I also got a nice "chat" with about EVERY one of the STP instructors... Hind sight is always 20/20 so just listen to all the good advice you got... there's plenty on here and I'm sure the STP instructors gave you good advice too. I use the harness turn ALOT and especially when I'm on my final approach (~300ft).
I took that canopy course with Shannon from PD about a month ago and definitely try above your hard deck to see/feel how your canopy reacts to your manipulations. When I downsized, my first jump was a high pull at 10k and just practiced flaring with toggles and rear risers. Practice these in succession with (1) watching the horizon, (2) watching your canopy react, (3) feeling the difference between braking fast, medium, or slow, and then (4) do it with your eyes closed. Do that with flat turns and even one toggle turns to see/feel how your canopy reacts and what kind of decent rate you have when you're doing that. Another thing Shannon emphasized was that if you realize you have flared too high, don't let up, hold it until it is time to finish and PLF to save your bones! Honestly, one high pull made me so much more confident in my landing abilities after getting to "know" my canopy.
But yea, don't do any kind of turns that low to the ground unless there's an object in your way. :) Hope this is helpful at least a tiny bit! See ya around the DZ!