Ouch, pen tips probably aren't that expensive. Btw I did make a bondage game some years ago: https://www.deviantart.com/foramouthfullofballs/art/Ada-s-escape-2-0-Stealth-bon...
If you want to steal any idea from it, you have my blessing. It's not like I invented everything myself hehe.
I work as a software engineer in video games, if you have any tech/programming related questions you can also send me a message. Anything I can do to help fellow DiD game makers.