@precastic's answer above is, imo, the right idea.
Worth noting that in IDEA 2018.2 (and surely other nearby versions) there are default keyboard shortcuts for this: (showing Mac, see Code > Folding > Expand All to Level for your system):
Cmd+Option+Keypad *, 1 - expand all to level 1
Cmd+Option+Keypad *, 2 - expand all to level 2
...
Cmd+Option+Keypad *, 5 - expand all to level 5
Note: these are "second stroke" shortcuts. First press Cmd+Option+*, then release, then hit the number you want.