From a German friend here, who agrees that the German is archaic, and wonders if Bach took it partly from some older text. She regards dir as a dative reflexive that is unnecessary in making the translation.

Literal-
Jesus, (you yourself) prepare for me (now) the path,
My Saviour, select (elect, choose) this believing soul (Faith-full)
and regard (look upon) me with merciful eyes.

Into idiomatic English-

Jesus, prepare for me the path;
My Saviour, choose this soul, full of faith,
And look upon me with mercy.

I think I will go with that. My experience is mostly with technical German. One key to good scientific German translation is to leave out unnecessary repetitive constructions and that seems to be true with early literary work as well, although I would not attempt formal translation of that form of German.

For Leonore Bockemühl, I made the translation of her "Odontoglossum, Monographie und Ikonographie," 1989, Brücke-Verlag Kurt Schmersow, Hildesheim, German and English together in one volume. This is a beautiful book, color illustrations by the author of all the species, and the "bible" on that group of orchids.