Is saying "O Lord, relieve me of this worldly life, for Your Noble Countenance is better than this worldly life and all that it contains" considered an act of disbelief (kufr), and is it true that everyone who wishes for death has committed an act of disbelief?

Supplication (Du'a) is a great act of worship commanded by Allah, and it has etiquettes, one of which is not to transgress. This includes praising Allah with what He has not praised Himself, or making a comparison between the Creator and the creation. Therefore, one should not say: "Your noble face is better than the world and all that is in it," even if the intention is sound. Supplication should be made with the narrated (ma'thur) prayers, as that is more complete. As for saying: "O Lord, relieve me from the world," if it is intended to ask for death, then there is a detailed explanation for that. But if it is intended to seek deliverance from the trials of the world, then it is good.