As an Android security analyst, I've packed this solution to record internal audio. This solution is a bit invasive even though it doesn't require flashing or kernel modding. The alteration happens at system level, with dynamic code injection through the awesome Frida. We are injecting code that basically tells the system we've got the needed system permissions to record internal audio.
This solution is tested and works on Android 9/10 and it's supposed to work on any prior version and device.
Let me know if the app is not recording in your environment.
I'll spend some effort in the next days to provide more customization for bitrate, frame rate, codecs etc.