Is a Muslim sinful if they do not recite all the prescribed post-prayer supplications, and is the recitation of "Lā ilāha illallāh waḥdahu lā sharīka lahu, lahul-mulku wa lahul-ḥamdu, wa huwa alā kulli shay'in qadīr" considered among the morning remembrances?

The prescribed remembrances (adhkar) after the obligatory prayer are supererogatory, not obligatory, and there is no sin in omitting them. However, it is preferable to perform all of them, and then begin the morning remembrances, such as saying: "Lā ilāha illallāh waḥdahu lā sharīka lahu, lahul-mulku wa lahul-ḥamdu wa huwa alā kulli shay'in qadīr" (There is no god but Allah alone, without partner; to Him belongs all sovereignty and all praise, and He is over all things competent) one hundred times. The time for the morning remembrances extends until zawal (the sun’s declination from its meridian).