In my opinion, you need to take a different approach.
You wrote: “The real question is the following: I have been worried about what if Nibbana is not the final achievement.”
That means you have not understood the “wider worldview” of the Buddha.
– If one has understood the process of getting to Nibbana, one would have absolutely no doubts about whether Nibbana is the final achievement.
– That means comprehending the Paticca samuppada processes.
1. If you do not have doubts about the rebirth process and the laws of kamma, that is a good starting point. That is why I asked that question earlier too.
– Because the key premise of Buddha Dhamma is that there is a rebirth process involving 31 realms, four of which have unimaginable suffering.
– Paticca Samuppada describes how rebirths in different realms arise, starting with “avijja paccaya sankhara.” That ALWAYS ends up in “bhava paccaya jati” and “jati paccaya jara,marana, soka, ..”, the “whole mass of suffering.”
– There is no jati (birth) without old age and death. Furthermore, there is much suffering even during existence in the lowest four realms.
– Here, avijja is the ignorance of the Four Noble Truths/Paticca Samuppada/Tilakkhana.
2. My advice is to focus on that instead of Anapanasati.
– Anapanasati can be cultivated ONLY after getting rid of those doubts.
– Those doubts start to go away only when one starts comprehending the Four Noble Truths/Paticca Samuppada/Tilakkhana.
– As I pointed out today in another thread, getting rid of sakkaya ditthi is the key to the Sotapanna stage. Together with sakkaya ditthi, two other samyojana (vicikiccha and silabbata paramasa) also break. Vicikiccha includes doubts (such as the one you have) about Buddha’s teachings. Silabbata paramasa is the wrong view that one can attain Nibbana by following rituals.
– Nibbana can be attained only after comprehending the “wider worldview” of the Buddha; there is no need for rituals.
3. My apologies for being blunt. In my mind, it is better to be honest, and straightforward.
– I can see that you have a good analytical mind. However, no matter how intelligent one is, until one learns the fundamentals of Buddha Dhamma, there is no chance of significant progress.
– Those fundamentals are the Four Noble Truths/Paticca Samuppada/Tilakkhana.
– I am not sure about your understanding of Paṭicca Samuppāda. You can take a look at the posts in the “Paṭicca Samuppāda” section. But, of course, it is up to you to decide how you want to proceed.