Is there a fatwa that recommends reading surahs of encouragement without those of deterrence in Ramadan, and what should someone do who commits a major sin, repents, then returns to it, and desires final repentance and steadfastness therein?

We are not aware of what was mentioned in the question about limiting oneself to the surahs of encouragement ( الترغيب ). It was common among the Salaf to frequently complete the recitation of the entire Qur’an in Ramadan, to the extent that some of them would complete it daily or every three nights. Imam Al-Nawawi mentioned that the Salaf had various customs for completing the Qur’an, ranging from a monthly completion to completing it three times a day. The preferred approach is for a person to recite the Qur’an frequently, as much as they can consistently maintain, without neglecting their other duties.

As for returning to sin after repentance, we advise you to promptly repent whenever your soul overcomes you, to resolve to remain steadfast, to strive against your lower self, to be keen on sensing Allah’s watchfulness, to frequently reflect upon the verses and hadiths of encouragement and discouragement, and to supplicate to Allah Almighty for sincere repentance. Remember the hadith from Al-Bukhari, in which it states that Allah forgives His servant whenever he sins and repents, as long as he repents with a legitimate repentance that fulfills its pillars: regretting the sin, abandoning it, and resolving not to return to it.