Is it permissible to say the supplication "Glorified art Thou, O Allah, and with Thy praise, there is no god but Thou, I seek Your forgiveness and I repent to You" after finishing the recitation of the Qur'an? And is there a specific supplication to be said after the recitation?

The expiation for a gathering (Kaffarat al-Majlis) – "Glorified are You, O Allah, and praised be You. There is none worthy of worship but You. I seek Your forgiveness and repent to You" – is recommended at the end of every gathering, whether it was a gathering of remembrance (dhikr) or a gathering where ill was mixed in. Thus, it affirms good and atones for ill. There is no specific supplication proven for the ختم of the Quran, but attending a gathering for the ختم of the Quran is recommended, as Anas gathered his family and children when he would complete it.