Not the solution, but you can use debug key for signing release builds to avoid blocking the installation from Google Play Protect. It looks like Play Protect doesn't warn for builds signed with automatically generated debug.keystore.
Note that your debug builds are not unsigned, they are just signed with a debug key.
Of course, you cannot use the build for production distribution (Google Play, Amazon, etc.), but it's still worth for pre-production internal testing which requires a high-frequency feedback loop.
You can add a task to build release with debug.keystore by adding the configuration in build.gradle, something like:
android {
buildTypes {
// add after the `release` definition
releaseDebugKey { initWith release }
}
signingConfigs {
// use debug.keystore for releaseDebugKey builds
releaseDebugKey { initWith debug }
}
}
then execute ./gradlew assembleReleaseDebugKey to build a release build with debug key.