You may need to make explicit what he will do when he has come. Alternate translation: "who will preach to you after I am gone" (See: [[rc://en/ta/man/translate/figs-explicit]])
Untying sandals was the work of a slave or servant. These words are a metaphor for the most unpleasant work of a servant. Alternate translation: "me, whom I am not worthy to serve in even the most unpleasant way" or "me. I am not even worthy to untie the strap of his sandal" (See: [[rc://en/ta/man/translate/figs-metaphor]])