Am I correct in my stance of declaring the act to be disbelief (kufr), without declaring the specific doer to be a disbeliever (kafir) in matters of apostasy (riddah), and leaving their matter to Allah out of fear of falling into the warning (of punishment)?

It is advisable to avoid matters of takfir (declaring someone an infidel) as long as one is prone to obsessive thoughts. Takfir of specific individuals is not asserted unless its conditions are met and its impediments are absent, and after establishing the proof (hujjah) and removing any ambiguity (shubhah). One whose Islam is established with certainty is not dislodged by doubt. Among the Ahl al-Sunnah (definition) wal-Jama'ah (Sunnis), takfir is not taken lightly. However, if someone commits a statement, action, belief, or doubt that constitutes disbelief and expels them from the faith, and after the conditions are met and the impediments are absent, then a scholar or judge may rule them an apostate.