I am self taught regarding drum set and have neglected training hihat foot independence. I can keep with a metronome alright so the sense of timing is not much of an issue, but when I try to open/close the hihat on 2s and 4s, everything falls apart. I can play both hands and hihat foot independently but no kick, or I can play both hands and kick independently with no hihat. What are the best techniques to fix this?
I'd recommend that you include your weak foot in the regular exercises from the beginning. Don't neglect it for 20 years and only then realize how important it can be.
Even though you may never actually play double-bass, I'd recommend to practice double bass exercises for coordination. The weak foot is considered "weak" for a reason. I found that I did not have control over it. After going through some double-bass courses, my whole posture adjusted.
Also, try to lead with the weak limb (foot or hand) in order to improve the control.
Using a second bass pedal instead of hi-hat helps because there is no difference in the sound and the technique between the two feet.