I see the obvious use of repetition but I felt like there might be some more specific type of repetition/technique to comment about here
Euphemism. By ‘thus’ Macbeth means ‘king’, and ‘king’ would fit the rhythm as well as ‘thus’, so it is significant that he avoids the word here.
Hyperbole. To be king insecurely is not ‘nothing’ as Macbeth claims; merely less desirable than to be king securely.
Feminine ending. ‘Nothing’ ends the line on an unstressed syllable, perhaps indicating Macbeth’s dissatisfaction with the idea.
Metrical inversion. ‘But’ begins the line with a stressed syllable (a trochee substituting for an iamb), emphasising this clause, in which ‘but’ is used with the meaning ‘unless’ or ‘without’.