I made a similar app for both iPhone and Android called Note Flash. Works with microphone and MIDI:

https://play.google.com/store/apps/details?id=com.pranapps.n...

https://apps.apple.com/ca/app/note-flash-music-sight-reading...

Musical Notes is a simple open source option

https://f-droid.org/en/packages/jl.musicalnotes/

https://codeberg.org/jonas-l/musical-notes-android

I have been using https://www.sightreadingfactory.com. It is available as a website and works differently from these apps

I get a lot of requests for an Android version. I've never had time but I hope there is something good out there

I made a webapp a couple of years ago that works in most browsers, including on mobile:

https://www.babeloop.com/

Not an answer to your question but I have 8 iPhone app on AppStore, and many more on my phone not published. Made an Android app, but it’s hard to publish to Google Play compared. Stopped almost at the end in the approval process, because I felt like this platform must really hate developers. Or is it just me?

When I see the stats on paying Android users compared to iPhone, I feel I always will start on iOS and only maybe make to Android if it’s successful somewhere else first.