From my experience, 4-5HG can work pretty well, but it does take time in the cylinder. I’ve gone as high as 15HG in the past.. And found that even 6-7HG can cause a blister. It’s better to go longer with less vacuum, IMO.
Warming up before you enter the cylinder, and maintaining that warmth is ideal. Your lamp may be either too hot, or too close. I like to wrap the cylinder with a gel pack. There are other options - do some research here. I think Titleist said it well - (and I’m paraphrasing) if your balls are hanging, the heat is right.
Stretch before, stretch between, stretch after.