Hey there Hugo, sorry for not seeing this earlier. It is a little difficult to say without seeing what you are doing however, a recognition of the problem is a great start.
So, if we get the magnifying glass out, where does it star?
When you do the Body’s Concert Hall Breath with the finger, in and out, is there any closure?
If not, mmmaaaahhhhoooohhhfffff with the tissues, is there any closure?
If not, with the visualiser rim or 2 fingers on the aperture corners, is there a back up or closure.
When you add the mouthpiece (no buzz) does the resistance of the cup create a build up.
When you add leadpipe, do things back up?
As far as the slingshot and/or aRRRRticulation goes, have you tried stopping the purely with the lips keeping the tongue out of the way? If you slowly close the lips to the point of complete closure simply using PASSIVE REDUCTION, can you keep the throat open?
It’s a really deep process finding the triggers. This needs to be explored away from the instrument #WindWorksSecret – see below.