Does the violation of some of the Prophet's Sunnahs, such as the method of Istinja (cleaning oneself after relieving oneself), affect the reward of a believer who performs the deeds of the people of the highest Paradise?

Cleaning oneself with the right hand is considered makruh (disliked/reprehensible) by many scholars and haram (forbidden) by some. Therefore, one should avoid it and not be lenient in doing it, in order to avoid scholarly disagreement. Cleaning oneself with water is preferred over using stones (istijmar), though istijmar is permissible and was practiced by the Prophet, peace and blessings be upon him. Combining both is even better.