Instead of all the pop ups, or ads etc, how about just a simple message to users asking for a contribution. It's a much more direct, and far less abrasive approach, plus it's personal and seems reasonable. I'm betting it will convert at 5 - 10% of users aged 16+.

E.g. Hi, I hope you are enjoying this app and finding it useful. Could you support this app by paying a one-time fee of just $1? It will allow me to keep the app updated and best of all running ad-free! Thanks in advance.