For thumbnails, I’ve had good experiences with https://github.com/artyom/smartcrop as used in my own https://github.com/fazalmajid/hugopix
It’s fast enough to generate thumbnails for my galleries, certainly compared to https://github.com/GjjvdBurg/HugoPhotoSwipe that is abysmally slow because the smart crop is done in Python, but I wouldn’t want hugo to be generating thumbnails using smartcrop every single time I run it.