i wanted to wish you only luck. Would the nur be considered to be a modifier for wanted?
Technically, it could be both. But in this phrase, it is understood to modify the 'I wanted'. So, 'I wanted to wish you only luck' - although technically possible - is most probably not what the speaker of the original phrase tried to express.
What about "I only wanted to wish you luck."
That's also 'Ich wollte dir nur Glück wünschen'. The difference between 'just' and 'only' can be conveyed via intonation.