Spending on a wheel is called RNG, you might get it, you probably won't.

Saving for 20,000 coupons is recommended because that is how much most of the high tier ninjas cost (6 paths Naruto, edo itachi, Konan angel of God, Minato edo tensei, sasuke rinnegan). Best events to use those coupons are those that give rewards besides the ninja you are spending towards. So events like lucky stars and lucky dice, and to a lesser degree mission mobilization. There is also an event called fukurokumaru deals that gives a rebate for coupon spending and usually has a good ninja in it.

Ninjas to go for, 6 paths obviously, edo minato, are some good ones that are at 20k. Edo deidera is a good one if you go for 180 fragments for his skill breaks (80 fragments to recruit, 100 to 4 star and then use advanced vials) but he costs around 35k all together.

Yellow scrolls, seal scrolls? Best to save up to 300 and use them in the Kage, Gnw, or the jinchuriki temporary treasures.

Shisui is still a good ninja with a good mystery, good barrier, and dodge. He also costs around 10k coupons