I’m glad for you that you are trending towards something that seems to be working for you. But, I’m fairly certain you understand that if you are trying to train according to the principles laid out in Forever that daily 20 minute HIT sessions would be akin to coloring outside the lines.
You might want to be open for considering that your conclusion,
“Can’t handle any volume with heavy weights” deserves the asterisk “Can’t handle any volume with heavy weights when I do HIT 20 minutes per day, my LISS is MISS, and I do compound lifts as assistance”.
Not that there is any reason you have to do heavy volume with heavy weights if what you are doing works for you, I’m just trying to highlight that you might be walking around with a belief about your own capabilities that might be “wrong” if the context within which you experience that inability changes.