Frontside spins can be a bitch to overcome this "opening up" issue. I still do it and loose all of my rotation. As you spin, it is so easy to open up at the first 90 when you are facing down the hill. Even a slight hesitation here will slow rotation to the point that you do not get all the way around.
Your setup sounds good (although popping off of the toes for a frontside gives you some extra pop but it is a hard thing to master and your timing is crucial). For now, stay with popping off of the heels and try a little more prewinding to help you spin a little faster and keep your chin planted to your front shoulder through the spin.
I thought popping off your toes for a FS spin was an ally oop and I think those are much harder to rotate I've done a few ally oop FS 180s and they just felt awkward. I realized one thing I do when I go for a 3 sometimes is that I'll open up before I jump. I've been practicing my 360s on a very small jump just so I can get my body to memorize the motion for the bigger jumps. I think the hardest part for me is not opening up at 270 because I have my FS 180s on lock i can do them off the biggest jump in my park which sadly is about 10ft and I can do just about any grab with it.
can you spin 360 standing still and strapped in? Practice that.
yeah thats easy I can 3 onto box rails no problem but it seems like my body doesn't like the bigger jumps