Need some help with the F2P LRs

Hello there.
I've already maxed out all currently available event characters, and I've been told not to use my stones until july, so now I have a lot of time and options I know nothing about or have some reason not to do:

->I've been told not to grind story too quickly because it's a stone bank.

->I don't know if there's any risks to grinding db story, so I might do that if this thread doesn't get any good answers.

->I do Extreme Z battles every so often but they get boring quite quickly when too easy and quite tedious when I try the more recent ones.

I'm currently grinding super strikes and prime battles 'cause I want to get UR/LRs for the bonus drop rates they'll give me in future events, but I've noticed a lot of the prime battle LRs have EZA zones that simply don't show up.

I haven't seen them show up on any one specific weekday, or at all, really.

Are they monthly or something? I'm told they're not very good but I still wanna leave them nice and maxed out before I move on to something else.