I'm new here

There is no rule that says what color your hair needs to be when you are older, you should do what you want.  If you don't want highlights, don't have them.  You can darken your hair, but since you have been going to a salon, you should let them handle that.  It can be complicated to get from light blonde with highlights to brown, so it will be a lot easier if you just let them do it this last time so it can grow out your natural color.  They will also have a better idea of whether you should get the highlights at the same time if you do still want highlights.  Getting highlights darkened and then rehighlighted would be very damaging...I don't know if salons would try to keep some of the blonde out of the brown dye to avoid having to do that (it wouldn't be easy so they may not) but you can ask them.

Yes, heat is very damaging.  Your hair will be a lot less dry if you don't use it. You may try reading posts on the long hair forum.  One has to keep long hair in good moisturized condition to keep it around for years and years, so you may learn some tricks to battle dryness there.