Idk if anyone suggested that but i have an opinion if you only want to focus on Nin. Create a patreon page, it have been on for some time and if anyone reaaly wants to support you might choose some payment plans you prepared and you will have your income from there while the game is still free.
The game will get some love and the ones that love your work will continue to support you from there so you can work on the game with full energy and full time. Also you will be able to put a team on the games development.
To honour the names of your supporters adding credits to share the names of them in the game somewhere, that a tough idk what will community thinks about it but i know i can spare few buck for month(Even if my country have a exchange rate of 1 to 7). Think about that it will be beneficial i see bimbos that make thousands and thousands of bucks just from nudity on patreon why you cant make a few bucks for your lovely game while keeping it free to play without p2w? @Ueda
Ps: Also i am thanking you for the game man, i am fan of naruto since 2012 but i recently come across your game its beatiful i hope you can work on it more while you are financially good.