You don't need to build using their build system to be available via f-droid. You can run your own repository and just provide the APK. If you want to be in the

You don't need to build using their build system to be available via f-droid. You can run your own repository and just provide the APK. If you want to be in their repository they will need to build your app. There also other restrictions to be in that repository like not being allowed the use of Google services etc.