The best tool I know of for simple conversion is Handbrake (https://handbrake.fr/downloads.php).
There is a Mac version that should accomplish the results you're looking for.
If your video is h.264, then you should only need to change the container format and copy the video rather than transcode it.
If you change the container (instead of transcoding), then it should only take a minute or two per video.
Handbrake is also capable of batch conversions.
If you want to script it, then use FFMPEG. I created a script to do this years ago: https://gitlab.com/ThatGuy/convert2mkv/blob/master/convert2mkv.sh
Just change the line: