The mavic mini doesn't use H.265 but it 's quite a high bitrate with maximum resolution 2.7k so an older computer certainly might struggle so dropping to 1080p and 30fps rather than 60fps will help.
It's likely that the transfer rate from card to PC won't be fast enough to play direct - so download the file(s) to the computer and play from there, playing direct from the SD card the data transfer could be too slow to play live no matter how you connect it.
It would be worth trying a different player. VLC is free and easy to set as your default video player, it will also come with any codec updates that might be needed.
Mp4 is nothing strange or fancy though