Sour is fine. In fact it means it is better, because of what you said. More bacteria, because these are good bacteria for our body. Traditional yoghurt is sour too. It is the last 20 years or so that big companies that sell yoghurt made their yoghurt less sour because they wanted to sell abroad, where yoghurt is not a traditional and well known food. Still what you describe sounds more than kefir or ayran than yoghurt. Which is perfectly fine. I like all of these things