The best ROI for ingots to coupons is the monthly coupon, which gives u 6x returns.
For jonin medal, although it "only" gives a 2x return, there are other benefits, mainly time saving (with instant completion for the dailies, as well as no cd for ranked battles/coin trial)
You'll need to evaluate the cost of this and see.

Since you're new, whatever you do, dont spam/spend for ninjas. Its pointless atm. What I would do is slowly (this will take time and test your patience severely) ac*ulate seals and wait for the next treasure to open. Use the free credits in events to get mats that boost your power. (Ninjas will always get obsolete, power remains)

With regards to upgrading materials like refines, thread etc, just mass collect them and only use during the associated event. Equipment refine runes imo for f2p should be saved up till you can at least level an item to level 7 (since its easy to collect med refines and pretty impossible for f2p to collect advanced refines, level 7 item should be end game for f2p atm. Also this is due to the event only giving credits that will disappear after event, unlike the BA event where the rebate is thread, which means you c*e it whenever the event rolls along and dont have to save up till your end game level plan). Med thread can be farmed from doing SA difficult

Get into a good group, where you can get help with tactics and protection, though its pretty much chicken/egg with regards to them taking in a new player hehe, need to be strong, but easier to get strong in a group, but you might be lucky. Some groups give weight to a player with a good attitude too, its not all about CP.