"We want to do less."
Translation:Chcemy robić mniej.
that really depends on the verb and context. But often it would be like you've said. Sometimes "robić" would be about feelings about the activity. sometimes it would be about ability to do something.
both sentences sound natural.
no 1 is about total amount of food eaten in a day. no 2 is about eating less all day less meals/smaller meals/less snacks.