Answer: In the name of Allah, Most Compassionate, Most Merciful,
Assalam’aleykum, I pray this finds you in the best of states.
Jazakum Allah khayr for your question!
Remaining motionless between the two prostrations
It is necessary (wajib) to remain motionless in the sitting between the two prostrations for the duration of saying a glorification (“Subhan Allah”). [Radd al-Muhtar; Ascent to Felicity]
Another strong opinion deems this to be an emphasized sunna. [Radd al-Muhtar]
This position could be used to avoid worrying about mistakes in the past regarding this issue.
In any case, failing to do so won’t invalidate the prayer. Nonetheless, it will be necessary to perform the prostrations of forgetfulness if one forgot about it. Failing to do so willingly would entail a sin, and it would be necessary to repeat this prayer even though the defective prayer was valid the first time. [Nur al-Idah]
Raising from the first prostration.
It is necessary (wajib) to raise oneself from the first prostration to perform the second prostration. The slightest elevation is enough to fulfill this duty. But it is prohibitively disliked (i.e., sinful) to suffice oneself with this. [Radd al-Muhtar]
It is an emphasized sunna to sit down between the two prostrations fully. [Maraqi al-Falah]
And Allah knows best.
May Allah Most High grant you the best of this world and the next. Please keep us in your du’as!
[Ustadh] Sufyan Qufi