As a FOSS maintainer myself, I recommend you to charge (a small amount of) money for the app. People could always compile and run the app themselves still, so paying for the app is a clear way to support the project. I see that you have donation on your page, but it rarely happens that people donate. Just my 2 cents.
Thanks for the suggestion. I do plan to publish the app on Google Play as well. While much of the audience is FOSS-oriented, there are also users who aren’t familiar with open source or compiling apps themselves, like here: https://github.com/CompassMB/MBCompass/issues/48.
For them, donations are a simple way to support the project, and they’re definitely appreciated.