So, I tried to create a simple sample clip, in order to test out the possibilities with the rendering option, given by the external rendering extension you created.
So, the MP4 Encoding seems to be decreasing the quality of the video itself. I created a few clips, so you can see for yourself.
I really enjoy messing around with this asset inside of Unity. Would be great if you are able to fix the Encoding / Buffering issue with the rendering!
Please note that the MP4 format, is there only for previewing purposes. For high quality rendering, please use either PNG or EXR Image Sequence format since those two are much better than the MP4 format. You can then make a video out of an image sequence in pretty much any video editing software including most of the free ones as well.