If you are willing to try an app that is neither open source nor available through an app store, I would strongly recommend to try the unofficial port of Google’s camera app for their Pixel phones. You can see some photos taken with FP3s or FP3+s using this app here:
In my opinion, the difference the app makes is so much bigger than the difference between the FP3 and the FP3+ camera hardware.
Pay attention to selfie photos when you use this app – on my FP3+, this app only uses a fraction (4 MP) of the selfie camera+ resolution (16 MP). So for selfies, you might still prefer another app.
But for the main camera (rear camera), the photos are really better than with the default camera app.