It seems there are many approaches to calculate the speed of Sun (for instance: Oort constant, CMBR, and others). Instead of having a "correct answer", what about "survey answers"? Should I change the question (edit / reformulate), or should I answer my own question and post another one? The only answer until now would not be disturbed, I think. In Astronomy SE are there survey questions? For instance: "Do you know another approach (for calculating the speed of Sun)?"
Oh, this is an interesting meta question!
Generally we are very careful not to change a question once an answer is posted, but I agree that what you are proposing doesn't really detract from or negatively affect the answer, other than preventing it from becoming "the" (accepted) answer.
Generally list questions1 are discouraged in Stack Exchange; questions requiring a long list of things as answers, as the answering can become unwieldly.
However there are plenty of well received exceptions to this, and it turns out that SE encourages questions that have many answer posts; multiple answers per question is even used as a basis for site graduation!.
So in this particular case for this particular question and topic I personally feel it will be fine to adjust your question to something like
What are the different approaches that can be used to determinee/estimate/calculate X?
or similar. You can leave a note under the current answer letting the author know of the adjustment (they don't get a notification) and explain that you appreciate their answer and it made you realize xyz and so have adjusted the question.
1For more background and discussion and thinking about "list questions" in the main SE meta:
Your question is not "what is the speed of the Sun?". It asks for the "speed of the Sun around the Milky Way galaxy", which is a specific question and should not be changed now there are answers to that question.
The "Oort constant" method isn't a different method, since the Oort constants have to be determined. The "CMBR method" doesn't tell you about motion with respect to the Galactic centre and would be an incorrect answer.
There are correct answers to this question but you haven't got one yet (though one is pointed out in comments).