Up to you. You can either piece other ninjas just to get them for collection purposes (and coupon recruitment bonus/ ranking) or keep on getting frags over 5* to get more iruka/scrolls faster. After a certain time, you will end up getting all those ninjas to 5* (eventually, some might take a lot of time) but there's no bad option there. Check if there's any pain/akatsuki member you would like to use in the future (check if they have future skillbooks that could fit your future team), if not you can just as well focus on getting more fragments for refining.

You will get almost all free ninjas from survival over time, all those that drop 30/80 frags. It just takes time. Also in black market, I tend to buy all 3 gold items, doesnt matter what they are. The longest time will take genins (ino, shikamaru, naruto etc) since they drop only 5frag tops. But, in time you will also get their frags without sweeping. Most of us played for 6months, some even since release and we simply have all elite instance and shop ninjas (survival/ranked) at 5*, because at some point you simply have nothing to spend your coins on. Buying certain ninja frags/sweeping their instance only speeds up the process.

Already answered your question: if you dont need anything else from survival/ranked shop, sweep hinata/kabuto/sai/lee, because they give 10pts when refined and cost only 500coins in their respective shop.

As for sweeping- I sweep ninjas, that i want to have skillbroken in the future- deidara, shikamaru, ino, sasuku, tobi, tsunade or want them 5* and sweeping is the easiest option- ningendo, tobi/deidara before i 5* them. If iI have spare stamina and still choose to sweep elite instances, i go for any 10pts ninja, doesnt matter which one.

If you want power- runes, if summon- koh. Drawing super rare from summon treasure takes about a year+ of saving scrolls. Koh requires 288 000 coins (if you dont get any frags in the meantime) and Kirin as another super rare can be slowly pieced in lucky board and events, since he's been put there pretty often recently. If you start selling every rune and save every summon coin you get, thats still about 8 months of saving for that ammount of coins. After you pull super rare, 80frags are converted the same way as normal yellow summon, that is 4k coins. It's your call if you want to invest nearly 300k coins with a 50/50 possibility to get a huge disappointment later on (when dupe is drawn). That being said, koh is a really good summon that helps a lot in current high combo chase meta. Plenty of future ninjas have 30+ chases as well, so koh can be used there.

Propably the best thing you can do, is to get your runes to lv7, then save coins. If you pull koh, good, spend saved coins on either kirin (if u want) or runes. If u pull kirin, u can get missing koh fragments. This way if you pull koh, you wont waste ~300k coins.

You can pull whole kirin or koh, requires about 2k scrolls.