Cave keys: rebates gives you rune stones and also treasured tool essences. Make sure you do full rebate (1050 or 2100 keys)

Refine runes: also make sure to save up for a rebate to make full use of wish credits and additional rewards

Other than training pots (which have great value to gain power) those are the main two things you should spend you moon and sun scrolls.

Your treasured tool will give you great ini, put all your essences in one tool and give it to your P1 ninja.

I never spend on purification runes, you'll get enough from events, so make sure to purifiy your P1 ninja with gold ini stats.

You'll get additional ini from things like summon cultivation and myoboku trainings, but I don't spend moon scrolls on those.