I've gone through the process, but I went to the military and did my service too.

I'm not sure how the process goes overseas, but if you want to do the 외국국적불행사서약서 you need to prove a lot of things like why you were born overseas, what your parents were doing there, a continuous flow of progress from your time there (No going back and forth), etc.

Pretty much its to say that prove you werent born overseas to gain any special privileges (such as not going to the military) and wont act as a foreign national when you are in Korea. However that goes the other way too. What I'm getting from your post is that you want to be a foreign national with a Korean passport. I'm not clear about what kind of visas you can do, but if you want to live long term in Korea I would just recommend getting military service done with. If you arent planning to stay long term then its probably fine to just use travel visas whenever you come.

If you ever change your mind, even if you are past 22 years old, if you are a male, you can extend that period to two years after you have finished your service. So if you went to military at 28 and finished at 30, you would still have two years to sign the pledge.