well, i think the important thing to note is that bronzong must be able to take a focus punch if it's up against breloom.
so definitely go with relaxed nature. as far as evs, let me do some calculations.
with 252 HP Relaxed no DEF evs, breloom's focus punch (adamant 252 atk) does 65-76%. with the same setup but 252 DEF evs, you get 52-61% damage. i think 228-232 DEF evs is good enough to pair with leftovers. not many brelooms have max atk with adamant nature, but it could happen.
and as far as having to 2HKO gyara, it's not all that necessary. it can get gyara down to the point where dusknoir can finish it if it ever came to that. the importance of charge beam is to prevent stalling verses other steels, especially those like skarmory who will do serious damage to this team if starmie gets knocked out, which it eventually will trying to switch into STAB hits directed for someone else.