From reading the general view appears to be gain length, then girth.
If you make something fatter first, then it clearly becomes harder to stretch.
If all you want to do is add girth, then go ahead, but I’d expect it to get more difficult to add length after that.
In reality most people target both to some degree. At 5.7” you are already well above average. I’m not sure if many or even any add almost 1.5” of girth, the general view is adding girth is much more difficult than length, and most feel adding 2” of length is very good to exceptional gains.
I also wonder why you want 7” girth? Try creating a shaft of this girth and sticking it in your mouth, what I’m saying is your probably at that girth going to make it impossible for you to get a blow job. You are also likely to hurt a girl during sex. Many guys at 6” or above report problems with blow jobs etc, read around.
I’m not being funny, but I wonder of the motives when a guy says he wants to be so big that he denies himself pleasure?
Your target is your target, but maybe you should research first. Regardless of length, the survey I saw says on a scale a-e, where a is ideal, at 7” girth girls would rate you a c, simply because your cause too much discomfort. Actually, believe it or not at 5.75 you are rated B, so actually by trying to gain that amount of girth, you may feel bigger, but girls in this survey regard you as worse than when you started!
6” girth is such a popular figure for a reason. 6-6.5” girth is rated as the best for pleasure in the survey I refer to, which not only looked at vaginal response, but also the physical response of girls.
Also, I have almost as much girth as length, that makes your cock look short and stubby and I’d say that regardless of size. If you were 7”x7” you will still look stubby. I’d much prefer a better balance of sizes than being equally loaded with girth and length, it just “looks” better. :) Dicks should be rectangular, not square!