OP, investing in runes is always a good idea. Specially if you want to do pvp where initiative often decided the outcome of a fight.

The 900 cave key pack is not the best but not the worst either. It comes fairly often (every odd week or so). On the other hand, the tree event where you need to wait 3 days before ripping the rewards is the best value in terms of keys/coupon since it costs 799 coupons and gives you the same cave key pack.

Also use the cave keys during the rebate. If you are a f2p or very low spender, then use 200 keys during the rebate and start savinf for next month, its not really necessary to go 500 keys during the rebate.

Try to get lvl 5 or 6 initiative runes to your move 1 and 2 units and at least lvl 4 to the other 2. Then you can stop focusing on cave keys since other things will give you a better return for investment (refining/purifying, 8 gates, charms). Take into account that charms will boost the initiative/critical/injury/combo/control stats of your ninjas depending on the bonus you get through the runes and runes only. To further boost these stats (most notably) initiative, you c*e the refininf/purifying system, which also increases these stats but whose bonus are independent from your charms/runes. Finally its worth to mention that charms also give bosts to you HP/tai/nin/def/res stats depending on the lvl of your magatamas.

That pretty much summarizes the possibilities you have to boost your power. I'd recommend you focus in runes/charms while doing a bit of the rest at the same time. Then once you are satisfied focus on another system and so on. Dont neglect 8 gates in the process though, although the purple chakras are not great they are of use, specially early on and the gold/red chakra you get after a while does give you very nice bonuses.