I am a past sufferer of acne. Roaccutane did it for me. Nothing else works like it. And like everything, it's not a 100% cure (about 90% actually), and occasional a second course is needed.
The pregnancy improvement is hormone related. So, unfortunately temporary.
Acne is a medical issue. I always get a little bit concerned when someone is describing multiple large cysts. These will all leave scars, big or small. And yes, you may find things that temporarily improve it but it is highly unlikely you will be shut of it, if at the age of 40 you are still getting cysts, without medical treatment if some kind.
I can tell you what has evidence diet-wise:
Avoid dairy. If you must have dairy have full fat, contrary to Caroline Hirons advice. And, again contrary to her advice, organic milk makes no difference and skimmed is much worse for your skin. (It's to do with protein bound hormones).
Limit your sugar and fruit intake. Known to worsen acne.
Nuts, in genetically susceptible folk, can be quite irritant to acne-prone skins.
Low GI diet might help, but it will only help to a degree.
I would highlight the scarring acne causes, which can cause a lot of psychological distress. And again, contrary to Caroline Hirons advice, (I would take her advice with a pinch of salt), waiting until you face is covered in spots is ridiculous. Even a few hormonal big spots a month will leave a scar. Multiply that by 12 for a year, for what, the next 10 or 20 years? That's a lot of scars. Get referred early, or go private.
Cystic acne is a medical issue.