Also, do you do personal sessions? Or is this just a hobby?
Funny you should mention that, it's something I'm contemplating setting up.
If you can make the trip to Oxford at some point I'd be more than happy give you a session. Email in trust.
There are lots of other exercises we can do to coach the hip hinge and then integrate it properly with the squat, but it's quite technical. That video of you hip hinging is ok, but not perfect.
Ideally, you'd do a progression something like:
RDL with bar
Single leg RDL
Weighted single leg RDL
Good morning with a light bar (broom stick handle etc.)
Along with various bracing exercises, and stretching anything that needs stretching on the way (keeping our squatting end goal in mind).
That K-Star stretch you posted might be useful overall (and I'd certainly recommend you try it), but it won't directly help what I'm talking about.
The basic mechanical problem that you're teaching your body to overcome is how to maintain spinal stability when pushing into hamstring end range. There is a huge mental component to this, and yet it isn't as simple as "just sort it out".