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I really dont agree with everyone who is telling you to do paused squats as a remedy. I've done paused squat reps in every warmup set (alternating reps) for the last 12 months.. i can nevertheless goodmorning those paused reps no problem lol. It's not going to stop you from doing that per se. But it may help with nailing the descent, ascent can still be goodmorninged though.

One thing you might want to explore is bodybuilding for assistance work to fix weak points. Don't be dismissive, russian and chinese olympic lifters use BB assistance work regularly. I found doing cable rows helped with keeping my chest position out of the bottom position of a squat, previously it would drop. Never thought doing something as simple as a cable row would help with my squat but it did. Which reinforces my thought that it doesn't necessarily have to be the quads or hams that are the culprit (they might still be weak though).

I think it's still a matter of quad strength.

I never ever goodmorninged any squat, and I'm quad dominant.

It can't be the hamstrings, because when you're goodmorninging a squat you are loading up the hamstrings more, and you wouldn't do that if they were weaker.

^ From the video above I'm quite certain your problem starts at the descent. If you hold a ruler or even your finger to create a straight bar path you can see that you get off center as you descend so it's not even a weakness issue. On the way up, your body is simply recovering or straightening itself.

I'm on board with paused squats to remedy the squat. You can take your time and hit all your cues until it becomes automatic. I'd film from the same angle and just use bar path as a reference.

Edit: scratch 2nd paragraph, your hips don't go any further back on second look. I do think the width of your stance plays a role here.

Like you said there's 2 different theories on it. 1) Quads aren't strong enough, allowing the relatively stronger PC to compensate via a goodmorning movement or 2)hamstrings are not strong enough, cant hold back angle, give up and allow hips to shoot up with the squat being completed by the quads. Which of these is true? Maybe neither.

The squat is a full body movement. If some part of the movement is affected by a weak point in your physique or strength distribution then you should get stronger there, wherever there is doesn't matter, only that you're squatting correctly at a given weight and progressing up from there while maintaining correct form. It could be anything, upper back, abs, erectors, who knows. Maybe a bit from column A and a bit from column B, ..., and a bit from Z. You nailed the causality:  "however since i can sit rock bottom in a narrow grip overhead squat and only lose my upright torso on the way up suggests it's not my problem." - this rules out mobility and suggests a strength weakness. When the weight is lighter all of the muscles in your body are more than capable of lifting the weight correctly. As you add more weight, the leverages of the lift change and the demands on the various muscles also change (not necessarily proportionally!) which places more of a burden on some muscles to compensate for a weakness in others, hence departing from textbook form.

My favourite theory is that form has a strength component to it, if you are strong enough to maintain good position throughout the movement of a backsquat in lighter weights but can't do it as it gets heavier, it means at a certain point the weight overwhelms that weakpoint. Identifying where that occurs and grooving the movement just below that weight will allow you to overcome that weakness and improve your form beyond that weight.

If I was going to fix someone's GM squat .. i'd have them work up to a heavyish single, then drop down and do volume at just below their form breakdown weight, working to increase the weight steadily over time but being patient before adding weight. Maybe master it for volume (say 6x6) before incrementing and repeating the process. What i wouldn't do is do a "normal" squat workout and then do technique work with lighter weight -- at that point fatigue has already set in and you're going to have bad form even with lighter weight.

you are not strong enough for the weight you are using.

highbar near max reps always start to fail with the caving of the torso assuming you have enough explosiveness to even take it from the bottom to the 1/4 depth on the way up.
when you get stronger, the explosiveness from the hole will cut the time of the concentric. by the time your back is starting to cave in, the rep is almost done.

eat more, sleep more. add more weight. good job! 

paused squats are good. or just lighter squats done many times with a lot of intention about leading with your chest. grease the proper groove.

Haha thats fine

Well at walmart if you buy any shirts over XL it's an extra 2 dollars because I'm sure all that thread for us fatties cost that much to make our moo moos haha