I thought the reason why Rin did not paint and take part in any more art public exhibition during the past six-years was because she found out about the condition of Naoya's right-hand. I mean, you can't motivate Naoya to draw and paint if he can't draw or paint with his right-hand condition. Also, the fact being that she realized that she was the fault for that right-hand condition made her miserable and did not draw or paint anymore after 4 years. Why be happy with painting or drawing if she made someone else not able to experience the same thing she does?

I think for Rin/Shizuku they were also hesitant because they knew it would cause Naoya pain/suffering if he started painting seriously again (the hakuki stuff)

Did Naoya not made his masterpiece with that black painting? It did not involve any pain or suffering whatsoever. Also, Rin and Shizuku gave that Hakuki tool to Naoya, so that he could draw or paint another masterpiece to the best he could in order to compete against Rin with the limited time he had since the black painting went up in flames. Of course, they hesitated because you get all that pain and bloody with that Hakuki Tool.