This will resize width down to 1080p and in some rare cases might slightly adjust highlights. Instagram does this to videos anyway so there's no way around it.

Because of the width downsizing it's recommended to not use landscape pictures because they'll lose more quality (although it's faster)

Also recommended to upload the videos as downloaded, no other editing or message apps in between 🎬

Make sure in Instagram you have Settings > Data usage > Upload at highest quality