Don't worry about spinning fast or winding up...
I had the same problem, I had a huge block at 540
Whenever I would go for a 7 i would wind up super hard and huck myself and end up panicking and flailing all over the place and then i would only end up spinning 5s and fucking them up hardcore
Just come go to the biggest jump you're comfortable spinning 5s off of, stay calm, throw a tiny bit more then you do for you fives, but don't huck super hard, that last 180 really isen't that much more rotation RELAX and stay calm in the air.
Staying calm and relax in the air was the key for me
After landing my first 7 I had no trouble stepping it up to bigger rotations cause i got used to not stopping my rotation and just staying calm and keeping my body rotating.