Assalaam alaykum. I have a few more questions about ease in prayer. These answers will help me very much, Allaah willing:
1. In each position, does the praying person have to recite the required Athkaar (plural of Thikr [expressions of remembrance of Allaah]) while simultaneously being completely still? If a person recites while not being completely at ease yet, should they restart their recitation of the Athkaar?
2. If a person swats a fly or scratches his nose while reciting Surah Al-Faatihah, does he have to become completely still again before continuing recitation/moving on to the next verse? (same for prostrating, Tashahhud, etc.)
3. Is being off balance, or slightly rocking back and forth while bowing (or standing and so on) considered not being at ease in that position?
4. In the Tashahhud, if the back and head slowly lean more and more forwards, is the Tuma'neenah (state of tranquility) broken? Does one have to straighten their back and restart the Tashahhud or repeat the words that they recited when their body started leaning forward?
May Allaah reward you. Answers to these questions will be tremendously helpful, Allaah willing.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

It is clear that you do not understand the meaning of the obligatory Tuma’neenah. Tuma’neenah is the tranquility of the body parts, and the limit of Tuma’neenah is that each body part settles in its place for the least time possible (before one moves from one position to another), which is the time required to recite the obligatory Thikr (in that position).

What is meant by the body parts settling in their place is the stability when performing one of the pillars of the prayer. While bowing, it is when one’s hands touch his knees; while prostrating, it is when one’s seven body parts (the face, including the forehead and nose; the two hands; the two knees; and the edges of the two feet [i.e., the toes]) settle on the ground, and so forth. For more benefit, please refer to fatwas 338716 and  200531.

Some slight movements that do not affect the validity of the prayer do not prevent the Tuma’neenah.

All the movements that you mentioned, such as bending slightly forward or backward and swatting the fly away from one’s face and so on, are all slight movements that do not affect the validity of the prayer and do not prevent achieving the Tuma’neenah.

Allah knows best.