^ That is so kind I'm going to risk showing you wrong, after you've described things well already too. I think Urtripping's 'mistake' is to open up their shoulders frontside flip style which makes me think they probably have a good one whether they already know it or not but kickflip style is different, you want cross tension as Ok put it but you're trying to stay in line with the board so it shouldn't feel like trying to go off axis (at all).
Way I personally like doing kickflips is front foot first serves to keep the board flat so most of it is on the board just below the front bolts and at a very slight angle that's as minimal as possible but just enough so that as soon as you'll pop you'll be able to throw your ankle and instantly get the flip. The flip happens also in reaction to how you pop so while my front foot stabilizes the board my back foot positions itself for the corresponding leverage, at a slight angle inwards just to match and then if you get that balance right you should already feel it that as soon as you pop a straight kickflip will happen.
By open shoulders on kickflip people mostly mean the way you'd open them up on ollies, no more no less, you want some looseness and they matter but it's more of an axis thing and how you set up your upper body in relation to your now in place lower body. Like, how your back as a whole is aligned.
Back foot probably is what matters the most, you want the aforementioned angle but also pop off the center of the tail (not tip) with your big toe, that way feels like the orientation of the knee on your popping leg will assist and secure the stability and direction. Cross tension also comes from there and then all you need to do really is ollie, but prolong the ollie by extending your leg and it should just work. Keep far back on the board and throw the kick in front of you but don't jump through the motion with it.
You'll find your sweet spot for the flick with time but it also changes depending on where your back foot is placed and what it does which usually is what people with inconsistent kickflips do not get. When you start getting good ones, setting up feels like loading a spring and you just know (which becomes part of the fun).
Kickflippers to study: Quim, Donger, Carlos Young, PJ. Maalouf too because he really pushes the technique, you can just see the formula in plain sight with every one he does.
tldr: twisted body, chopped up feet.