The "plain speech" of the Friends has usedf 'thee' in nominative places for centuries. But it is a local variation, not the fundamental form, which is thou in the nominative and thee in the dative and accusative.
I would be tempted to use "thee" in the first line (May thee now rest...) rather than "thou" because it is a dative construction, in that the wish is being given the recipient. And it sounds better to my hear. Cf. "God rest ye merry, gentlemen".