It is common for discharge and odor to develop in certain parts of the skin such as the belly button, which have a tendency to remain damp and dark, which are excellent conditions for bacteria and fungus to grow. The most likely possibility is that you have a minor fungal infection of the area (yeast or "thrush" as it is often known).
The easiest way to treat this condition is to continue doing what you are already doing. It is especially important to keep the area clean and dry at all times, since it is the moisture that most encourages fungal growth. Some of the treatments that you have been using, such as tea tree oil and witch hazel also have antifungal properties. You may also try an over the counter antifungal cream if those are not working for you.
You should not have to see a doctor
for this condition unless it worsens. For example, if you have significant redness or pain of the skin in and around the belly button this could be a sign of a worsening skin infection. Also any fever
or pus-like discharge should be evaluated as soon as possible by your primary care doctor