It’s not open source but it’s quite literally the best Android application I’ve ever seen, by a significant margin. It’s from the good old days where apps were small, very well designed, very polished and did not collect data.

It’s FX, a gem ever more lost to time. The site is a bit old and unmaintained so SSL certificates may give you a bit of trouble, but you can just ignore the warnings, it’s just an info page.

FX, despite not receiving updates for years has so many features of such great quality, I’ve yet to find any alternative with a truly competing interface and feature set.

Been using it for 10 years and it has been the pinnacle of Android.

Edit: I know you’ll be detracted from trying it since it’s not FOSS, but seriously give it a shot.