US Visa was rejected for the first time andI want to apply again

From my title, you probably think that my problem is similar or a duplicate of some cases. But I don't think so. So, please share your thoughts :)

I am an Indonesian and living in Germany for 4 years. I have been doing my PhD since the last 4 years (with a scholarship BUT not as an employee as most PhD students in Germany do) and will have my defense in Sept 2019. My residence permit in Germany is temporarily valid until Jan 2020. In May 2018, I applied a US visa for the first time with a type of (combination) B1-B2. The reason I applied at that time was to attend a (non-scientific) conference, so it has nothing to do with my PhD. I have fulfilled all the requirements and was able to show (in my written application that submitted to the system) that I get monthly scholarship with a (more than) enough amount for living here. At the interview, everything seemed to run smoothly until the officer asked me about (1) my PhD topic and (2) my plan when I finished my PhD. I answered that (1) my topic was to develop a specific computer system (as it is what I am doing here) and (2) I will go back to Germany after my trip to the US and then go back to Indonesia after my PhD. But suddenly the officer looked unhappy and told me "Well, I think there is no such system in Indonesia yet and you can't develop it there". I only answered that he was correct but the applications of such computer system can be used for a development of hazard system in Indonesia. However, before I explained further, the officer said "I am sorry we cannot issue a visa for you". I received (as usual) a letter of section 214(b), which stated

• " ... you were not able to demonstrate that your intended activities would be consistent with the classification of the non-immigrant visa you applied".
• It was also written in the letter (as I think written also in most cases) that I do not have strong ties with the foreign country and no ties that sill compel me to return to my home country after the trip to the US.

Before I apply again this month for the same non-scientific conference, I would like to have your thoughts:

1. Would it be rejected again as there is currently no significant improvement in my application unless the date of my PhD defense?
2. Did I apply wrongly at that time that I should apply only B1 instead of a combination of B1-B2? I asked this because it is implied from the first point above.
3. Have I explained the officer about my PhD topic wrongly as though it is too sophisticated for a country like Indonesia, whose technology is currently increasing, so that even if I go back there it has no meaning at all and that's why the officer did not trust me? I asked this because it is implied from the second point above.
4. I still do not understand why it was written on the letter that there is no strong ties between me and the foreign country (which I assume here to be Germany, right?). I have explained, I am doing my PhD here and it obviously indicates the strong ties between me and Germany. Am I wrong?

I am just afraid that if I apply again this time and it is rejected then it becomes more and more difficult for me in the future. In other words, should it be better that I wait and apply when I am in Indonesia?

PS: A friend of mine, who comes from China and is also a PhD student with a scholarship (even with a slightly lower amount than mine) has also applied the US Visa for the same conference at that time and his application was accepted.