Depends on your power and on your level in rune stones, charms, refinements and magatamas and if you are f2p or not. Tell us those infos and we can help you :-)

Okey here we go. Lvl 80 39054 power. Rank 2 on my server, can beat everyone except the pay to win which is rank 1. All charms level 4 except 3. All refines level 5 except 3. 40% lvl 5 megatamas rest lvl 4. Tactic level 1. All runes minimum level 3. Mostly are level 4. Current formation made of Mei Watermain Sage Naruto and Ao.

How much coupons you have? I would wait and use it for lucky board since your power level can still be higher and lucky board helps out getting charms and refines. Players have stated that people should get clothing when they have power at least 60k. If you don't spend any cave keys for the entire month and wait for rebate, you can get close to at least 100 keys just by getting it freely via events so I would rather save the coupons if I know i'm going to get that much in the month.

2800 coupons now

Honestly you should level up your tactics, you'll gain quite a bit of power if you do, outside of that focus on your runes.

You should spend on keys, even if directly, if your runes are lv 4 or lower

You should wait for key packs and never buy individual keys (i.e. pay coupon to explore) if your runes are mix of 4s and 5s

You should not spend for keys except maybe the key mystery fruit if your keys are mix of 5 and 6

You should not spend on keys once your runes are all at least lv 6. (FYI, 200 key at this point will give you ~200 power @lv 5 charms, and this is during rebate)

Mind you, you should pick key over coupon in the wishing tree regardless.

For clothing, 60k is the bare minimal, I'd wait for ~70k.

Otherwise, coupon should go to lucky board.

I would suggest, given your situation, if you didn't do it, to bring your main character mood to +45% and the other 3 main team ninjas ones to 25%. After you did this you can bring to level 3 your main tactic (to 4 if you rank up tactic soon by luck) and with what is left you fully go in lucky board and with your redeem points you go first for charm packs and after it for medium refinements (in the range of level 4-6 charm packs give you a very good power boost/coupons spent ratio, but remember to use the charms only when there's the rebate). In the meanwhile you gather cave keys from events/wish tree and when you hit 200 you use them in the rebate.

FYI, if you're f2p, you'd roughly get around or close to 200 keys each month, that counts from trees and redeems through events.

Save your cps on board may be a better choice.

If you already hit 200 this event, save, wait and see how many keys you will get til next one.

This time I got around 210~ keys from tree everyday and through events and sometimes draw. :/