Hey dude. I'm a lvl 69 who started as a water main. I swapped to a lightning main to help do more damage. The truth is water main is used solely for healing which means you would usually need to have a rare ninja which you usually get acess too at lvl 70 through GNW treasures. (You get a few more types of treasures at 60 and 65 but it's recommended to wait till 70 for the better ninjas. because if you get a "bad" draw its still gonna be a great ninja war Neji or something as opposed to the normal variants you get. Late game gameplay = keeping thag one strong rare ninja alive to carry you. Which is why earth and water scale better into the late game.

Since you have GNW Sakura. As for lightning main you will eventually replace Hinata with kimimaro (given free) but he gets ridiculously strong at 3* which should be what you want him at. Your Sasuke also needs to get his stars to 3 at least for new skills.

As for ninjas in shop, I learnt this the other day but the best think we can do is buy the cheap fragments. E.g. Hinata, Kabuto, rock lee to help build them to 5* asap because you will unlock fragment refining which will basically give you a better shop Because you can trade the excess for rarw ninja fragments such as iruka. (My Hinata is only 3* now because I only learnt this two days ago but I did my research and this is the most "value" you will get.)