Thanks for the question! I am glad you like it.
I'm sorry. Unfortunately I have no plans to release a paid version on Itch.io. How to sign up for pixivFANBOX is difficult for me to explain in English, but it seems you need to create a pixiv account first. Here is a link to the help page in English.
pixivFANBOX
https://fanbox.pixiv.help/hc/en-us
There are also similar pages of mine on other sites. Please consider this one if it is easier to understand.