I have finally figured out how to do the GDJ using the guide, but it's wildly inconsistent and I wanna know if theres any specific strats for individual levels? The GDJ second jump timing changes a bit on different levels, and some levels just don't ever get consistent at all, I want to know if anyone has tips or advice for me? Or maybe I just have a skill issue and need to practice more?

The technique is annoyingly precise in a way that makes it practically impossible to consistently hit. While I myself am not sure on the exact mechanism of this glitch, nor what makes it fail, you can get, at the very least, more consistent at it with some practice. With the dozens of hours I've poured into the game, I estimate that I can get a successful GDJ about 60% of the time after I've warmed up.

As far as I know, the cooldown before you perform the second jump of a GDJ matches the cooldown you get from the Double Jump power-up/the stored jump when you fall off of a platform. The issue with "jump timing" stems from the fact that the platforms that speedrunners usually reach with GDJ vary in height, and therefore may require a later second jump to gain the height necessary to reach/vault them.

One example is the ceiling clip used for Level 9. When I GDJ to the pillar used for the clip, I try to jump when I'm at the apex of my first jump to maximize my height gain, as the pillar is hard to reach, especially when taking into consideration that you need to be crouched when you get near the walkway above, as hitting your head will most likely result in you losing too much height.

TL;DR: On occasion, we all feel cheated by the GDJ. Practicing helps, and you should study the jump timings needed to reach seemingly hard-to-reach platforms.