What's stopping clones from replicating your UI right now? Are you concerned with loss of IAP revenue to clones?
I would suggest you open source it under a non-commercial license. Also, take a look at how some successful android open source projects are run. In particular, I like XPrivacy (https://github.com/M66B/XPrivacy). This is an excellent app. Open source with optional purchase, similar to yours. Personally, I paid the $6 they charge because I'd rather not compile the src myself.
You've already had a lot of downloads and are an established app. Clones can't replicate that.. If you are a recognized dev on xda, with your own thread for discussion, feature requests, bug reports etc, no clones will be able to replace that aspect even if they can clone your app.
You should be able to get more downloads/purchases from privacy-aware users. And you can have donations for feature requests if you want.
I think the main part about being free software is not that the code could be reviewed or whatever, but that the software respects its users' freedoms. And for me, that alone is a good reason to respect the software and its developer.
XPrivacy is a great example. Another one would be "Conversations" XMPP client, that is FLOSS, but is a paid app on Google Play. Even though I wouldn't call it completely mature yet (it's 0.7.x and lacks some polish and minor features), it's a good app that does its job very well and 1-5k users (not bad for non-gratis app, I guess) seem to agree.
I would suggest you open source it under a non-commercial license. Also, take a look at how some successful android open source projects are run. In particular, I like XPrivacy (https://github.com/M66B/XPrivacy). This is an excellent app. Open source with optional purchase, similar to yours. Personally, I paid the $6 they charge because I'd rather not compile the src myself.
You've already had a lot of downloads and are an established app. Clones can't replicate that.. If you are a recognized dev on xda, with your own thread for discussion, feature requests, bug reports etc, no clones will be able to replace that aspect even if they can clone your app.
You should be able to get more downloads/purchases from privacy-aware users. And you can have donations for feature requests if you want.
Just some thoughts.. best of luck with the app!