I too suffered from cystic acne but for decades. I'll share my thoughts and experience on both meds and diet. FYI --- I am now clear.
MEDS --- like you I was prescribed many acne meds but this went on for decades. However it was always one or at most two at a time (i.e. an antibiotic or a topical or birth control, etc.) My current derm of 5 years who is an expert in adult cystic acne says that only a multi-prong approach will clear up resistant adult cystic acne. This means that being prescribed only one or two things at a time will not work and unfortunately this is what is usually done. To get clear you must do a multi-prong approach. You must use simultaneously something that opens pores + something that kills acne bacteria + something that removes dead skin + something that lowers androgen levels. For most adult sufferers it is a regime you must do forever if you want to be clear. Why? Some people genetically have pores that don't shed properly, in addition to being very sensitive to androgens, etc which makes the perfect environment for ongoing cystic acne. If you message me I can explain more. So the solution is to do things that help the skin behave in a more healthy way...without doing more harm (i.e. oral antibiotics). My regime is listed below in my signature but in summary it is using a BP cleanser, followed by a topical antibiotic. This combo is well researched and documented as being effective. I also take 50mg of Spironolactone which is an anti-androgen but is not a hormone itself. It just blocks androgens. I am actually weaning off of it as the other parts of the regime should keep my cystic acne under control. My derm has me doing once a month microdermabrasions (diamond tip...not the crystals) and this is to open pores and remove dead skin. It is amazing. All scars are gone and my pores are totally clear. This is essential to allow the BP and topical antibiotic to work well. The pores of people with cystic acne are usually so plugged up that meds don't get in well eneough to kill the acne bacteria. Please read the rest of my regime below in the signature area.
DIET - I have studied and tried many many anti-acne diets and have finally found one that really works. You are definitely on the right track. Please check out Dr McDougall's web articles on acne and diet. He advocates avoiding all fats, especially all processed fats. And avoiding dairy and flesh foods. He recommends eating whole natural foods such as whole grains, tubers, vegetables, and fruits. This is the diet of cultures that don't get acne. It is a sustainable diet and very filling. I love it. I admit that I personally still eat eggs and occasionally some wild fish. But my skin remains clear. For me what really got me clear diet-wise was avoiding all dairy, all fats (oils, margarine, etc.), fermented foods such yogurt, alcoholic beverages (I may have a bit of rosacea which is reactive to fermented foods), yeasted foods such as bread, vinegar (may be the rosacea issue) nuts and all sugary foods. My skin, digestion and energy levels are all much better...I feel great. I am 5'5" and weigh 120. I do marathons and hike all the time. Here is what I eat typically:
Breakfast --- huge bowl of oatmeal cooked with raisons, chia seeds and topped with fresh berries, banana and soy milk.
Lunch --- huge salad with fresh vegetables, potato or yam or leftover rice, any leftovers from dinner, etc. Or I make a vegie burrito with avocado and beans.
Dinner --- huge salad, lots of steamed vegies, rice or potato or yam
Treats --- my special homemade non-fat, non-sweetened banana apple bread
Condiments --- I like brewers yeast (it's not really a yeast but actually is a dried mushroom), garlic, cumin, lots of herbs, and herbed salt.
I am never hungry, eat tons of food and maintain a good weight. Love this diet. After about a month on this diet my usually oily skin was normal and my skin looked glowing.
I hope this helps you. I will follow your thread to see how you're doing. FYI -- diet alone may not get rid of really bad cystic acne. You may need to do a regime like I'm doing along with diet. And then after getting clear for a couple of years wean off any oral meds. With really resistant cystic acne you may have to continue with topical meds to stay clear.