I would go to 5-3 after this event and pick all the stage girls on the way the way to 5-3.
I stayed at 5-3 myself untill all but 4-5 girls was level 100.
I got bonus stage 3-4 days ago.
Btw I have discovered that there are 3-4 girls that can only be obtainded GHQ - Strange.

There is one girl in PeroPero that players could get before a bug fix but who was removed after fix.
Very few players was Lucky enogth to get that girl - now it is imposible to get her.
I think the bugged girl should be added to N-Gacha so all player have a chance to get her.