Is the purpose of Salah fulfilled by our immature style, where in the whole Salah, we are always engrossed by millions of thoughts? Secondly, is there any specific Quranic verses or Ahadiths regarding this issue?
The purpose of prayer is to remember the Almighty so that we can purify our heart and worship Him in the best possible way.
You rightly stated that people are generally processing many thoughts while in prayer. In fact there might be only very few who read their prayers with full attention. I would like to argue that this is not entirely human beings fault. After all, our brain is working non-stop and it is not always easy to control it. I do not think that the outcome of prayer is either one or zero, meaning that we either need to read it perfectly to reach the objective or otherwise we will not get benefit from it at all. I think every person gets benefit of his prayers proportionate to the effort that he puts in his prayers. What is important is to do better and improve rather than staying at a same level.
I hope you find these helpful.