I would say it is worth making the changes you want to make before starting a cycle asap, if possible. Whether that be diet and/or supplements I understand it can take up to 3 months to see the benefits (if results are truly linked to certain supplements or diet, it is difficult to be sure if you try everything what truly makes the difference).
I had a cycle in April/May and ended up having an abandoned and failed IVF cycle with the NHS-I had just started supplements 3 weeks before starting IVF such as high dose of CoQ10 etc, and through the cycle I had 4 follicles, and ended up with 2 mature eggs of which led to failed fertilisation. I do have low AMH, so didn't expect much but it was still disappointing nonetheless.
3-4 months of high protein, low carb diet with the a mixture of supplements I picked would be best for me, and I've just had an egg collection where I had 8 follicles, of which 5 eggs were collected, 4 were mature and 3 fertilised (would have been 4 but it degenerated through ICSI which is a risk with ICSI). It is still very early days as to whether things will proceed further but whatever happens it has been an improvement in terms of follicles/eggs/fertilisation.
Now I don't know if this is also attributed to a different IVF protocol as I was on max stims earlier this year as this time I did natural modified which is gentler, but as I said it is difficult to ascertain exactly what makes the difference or whether it is just luck of the draw with each monthly cycle when follicles are naturally recruited or if the diet/supplements changes make a difference.
It really is personal preference. My thoughts are if I'm doing this then I wanted to chuck everything at it, plus the kitchen sink so to speak as I'm also doing acupuncture and reflexology. Whether it helps who knows, but I feel like I'm doing something towards it-just helps me personally on this inevitably difficult journey!
Sorry long winded response!