Seal fragment shouldn't be prior item btw i mean yeah you cansave a lot of seal and get ninjas like susano itachi hashi edo madara etc etc but they aren't that op or like can carry you through battles priority should be yellow and purple refines only i always buy yellow refines (my power is 300k) there aren't many options for us to power up in late game (for f2p) it takes a lot of time so it's better if you start saving from start

I was thinking to get SR in GNW treasure and then save up for refine runes and charms perhaps

Because I really don't have so called P2W ninjas, only masked man

Well it takes a lot of time to get the SR ninjas from treasure and as i said they can't carry in a battle so the best choice to get a op ninja is you save 25k coups and wait for fuko rebate then spend it on good ninjas like Kushina,Minato edo, killerbee 8tails, kakuzu they all are really good and actually can carry whole battle(not sure about mianto tho) (also keep in mind it should be 25k rebate not 30k) and spend the moon coins on powering up you'll get seal scrolls from group either way powering up takes a lot of effort and saving if you are f2p