@sbmfj my pleasure, I remember when I was a kid that trick and regular stance frontside flips were a complete nightmare to figure out, at least now there's all the info you need (and then some) online indeed.
I think this trick is supposed to pop kind of high actually, in fact the pop is so vertical for a split second (even on horizontal hardflips that end up looking like frontside varial kickflips - mine are like that), it's one of the rare tricks where I'll put most of my energy into it, the pop. Like, I'll make sure my foot positioning is correct and secure with the front foot where it needs to be to be ready to react (to be accurate, what works for me is to have the toes on my front foot in the very center of the board for this one), and then when I do pop it's straight down and hard, having your back foot in the pocket is what will cause the start of the f/s shove-it rotation and then also your front foot dragging the board over to the side - I'd say it briefly acts like a guide before you get to just brush off the concave with that lateral ankle flick. You have to jump but your movement doesn't have to take effort as long as it's accurate (that applies to all of skating to be honest), hence why the flick sort of feels like a light side step, and the whole trick (at least the horizontal way) almost feels like you're flinging it ahead of you before it comes back around and you can easily catch it, so if anything the worst thing you can do is hunching over your nose. I'd say maybe adjust your back foot positioning to figure out the sweetest spot on the corner of the tail that will allow you to get the full spin right even if that causes you to miss the flip and catch the board upside down, and then your front foot accordingly so that it directly reacts to the pop and does what it's supposed to once you flick off the side. If you're underrotating them then it sounds like the problem is in your pop, there's probably a way you could optimize your effort by really finding that sweet spot, and make sure your shoulders remain open too (you shouldn't throw them around like for a frontside flip is all, just face forward the whole time).
For varial heels I just gave some tips to Skatebeard earlier, that trick is a lot less awkward than hardflips, again it's all in the right foot positioning, you build tension in the pocket of the tail (heel-side this time), you pop a normal frontside pop shove-it this time with your shoulders always aligned and parallel with the board (don't turn your shoulders) and you kick out, extending your leg that way is a lot more natural than the opposite way for a hardflip. Angled front foot tip I gave Skatebeard still applies. In both cases though, focus on the pop/rotation/catch and then getting the full flip sort of comes second, I'm sure you'll get both in no time.