What is the ultimate solution for someone who sins frequently in Ramadan, despite their diligence in worship, and fears that Allah may not forgive them or free them from the Fire?

It is not possible to definitively affirm the acceptance of good deeds, the forgiveness of sins, and emancipation from Hellfire, as these pertain to the Unseen. However, a Muslim should perfect their worship, be sincere to Allah, and avoid prohibitions, then hope for acceptance. Repentance is obligatory whenever a servant sins, even if the sin is repeated, and Allah accepts the repentance of whoever repents and is sincere in their repentance. As for definitively affirming the acceptance of a specific person's repentance, that is not possible, and the servant remains between fear and hope.