So is this يا basically a vocative particle, as in Thanks, O Rawad?
Looks like it..
Could a native or experienced learner explain? I would also like to know.
According to this: https://www.al-islam.org/elementary-arabic-syntax-2-rashid-al-shartuni/vocative , yes.
Yes. "Yaa rawad" can be translated to "O rawad". You use "yaa" before the name of the person you are talking to.
I looks like standard, but in everyday language they dont use يا - it will be just Shukran Rawad. I am not a native speaker but i live jn an Arabic country, nevee heard it like this...