OBS Studio is for interactive use. Someone sits in front of a PC and acts as kind of director to create a live stream, and manipulating it while streaming.
It seems you just want to add some static info to some static camera - no need to do anything interactively. In this case, OBS Studio is not the best tool. You need a tool that allows unattended, automated usage. Ffmpeg is such a tool. It's a command line tool, and it can receive video and image data, merge them, encode it, and send it to some streaming provider. However, it's beyond the scope of this forum to figure out corresponding command line options for implementing your specific requirements. It's medium difficulty: you need to define 2 input streams in ffmpeg, merge them, and output the merged streams to a rtmp destination.
About quality: there is always quality loss with recoding. If you want to add something to a video, the best way to do it is before the video is encoded, so it doesn't need to be encoded twice. If the camera is a blackbox and outputs only encoded video, you have no choice to decoding and recoding.
The load of encoding is a common challenge with all video encoding. You need proper hardware that is powerful enough for encoding. You have to make yourself familiar with the computing power required. Rules of thumb: doubling the resolution requires 4 times the computing power, doubling the fps requires double the computing power. Or the other way round: half the resolution requires a quarter the computing power, half the fps requires half the computing power.
