Parts of prayer that are not obligatory

1. It is possible to dispense with the majority beyond the two blessings (Baruch, who said and praised). It is also desirable to have another chapter in the middle (Asheri). And evidence for this is that those who are late for the Bi'achn can skip, and according to the method of the Ge'onim (and so does the Mishnah) there is no need to complete it later.
2. Greetings to the Lord are obligatory.
3. Rivash Si' Tiv wrote that Tachanun is only a custom. Rabbi Ovadia (Yabia says Ch. 3 Si' 11:8) permitted those who are in a hurry to work to forgo it. As is known, the people of Israel are tolerant of this.
4. A custom in the Hebrew language (Yerushalmi Ta'anit 4:5). There it means that one wonders whether it is even permissible to say it without consent, and the conclusion is that it is permissible and certainly not obligatory. But it is noted that there is such a custom.
5. I think that this is simply a duty (and the frame of our lips is complete). The Father of the World is accustomed to overlapping, and I do not know the source of this.
6. Simply put, it seems that it is not an obligation but a custom. Only during the observance of the Holy Days is it part of the prayer regulation itself.

But it should be known that customs must also be observed (do not neglect, vow), and therefore reasons are needed if one wants to overlap. Simply put, the dissolution of vows is also required (unless the custom was based on an error. Here I do not think that is the case. Even if you thought it was a halakhic obligation and it turned out that you were wrong, the world believes so and I do not think there is an error here that completely uproots the vow without the dissolution).