is it pronounced /ʁɛst‿i/ or /ʁɛstz‿i/ or /ʁɛstəz‿i/?
It is pronounced /ʁɛstəz‿i/. I guess it could be /ʁɛstz‿i/ in fast-paced speech, but that /stz/ cluster feels a bit too awkward to me.
Notice that an extra -s is added to the imperative before y to avoid pronouncing it as /ʁɛst‿i/ which sounds really unnatural.
It is also done with en, like in "parles-en" ('talk about it'), pronounced /paʁl(ə)z‿ɑ̃/
so is it the "standard" pronounciation to pronounce the e in this case? (like for example just "restes" is pronounced /ʁɛst/ but some people pronounce it /ʁɛstə/)
La troisième. Enfin moi, j'utilise toujours la 3e. Trois syllabes nettement articulées.
En langage très relâché, si on est en colère en plus, on peut dire rès-zy, en omettant le "t"
actually it says ʁɛstə-i there
but that's not a very good source, for example I tried "vas-y" and they say it's pronounced va-i (correct is vaz-i)
No not exactly .
In restez-y the second e is pronounced like é and then zy . Like mangé, jouer, nez , parlez, dé, fiancé...
In restes-y it's eu and then zy . Like in jeu ou je , peu, lieu, te, de , deux ...