To stop the tide mark water the wash down more with Lahmian medium and apply a couple of coats, regarding contrast don't be scared to go bold with your highlights, a tiny spot highlight will make the viewer focus on that part so use it to draw attention to a particular area.
You don't have to use green to highlight green, experiment on a test model, using say yellow, white, orange will add a dynamic to the model. Experiment is the key.
Watch lots of videos from different painters and find a style that suits you and copy what they do.
W&N Series 7 are my goto brush they are amazing, expensive but worth it.
I think you have done a very good job so far and pushing yourself is a very satisfying aspect of this hobby, keep it up.