Thanks.

Not through mpv, but if all of the videos are the same resolution and codec you can use ffmpeg to concat them without needing to re-encode anything.

It could be worth making a little wrapper script for this, but the ffmpeg command itself isn't too crazy. You can Google for "ffmpeg concat multiple videos into a single video without re-encoding".

ffmpeg is super powerful, you can technically resize and alter videos into a consistent format and then encode a new video too.