Using Rive for Captions on captured video clips (Flutter)

I am defining a spec for a flutter App and was suggested to use Rive as the tool to provide animated caption graphics to be captured on video clips recording on a smartphone.

I can add the text at runtime, so my question is about how to combine the camera feed and the animation into the same output file saved to the device. Am I reliant on screen_recording or is there a better method?