No maintaing is a separate action from attacking, wothout greater grapple you cant attack while maintaining. Best just to pin.
I would suggest using the snake shaman, it is what i am currently playing and by level six i get to be a huge snake with a bite attack that leads to a grapple (with the grab ability) that has a plus 21 grapple cmb. All this comes with constrict so apply bite damage again on successful grapples.
Bears make good tank druids