Generally websites like AMR uses a baseline for every class and not character individual.
So i'd say ignore what AMR says and just go for mastery wherever you can, or use simcraft to get more in detail for your character.
(im BiS geared and it's telling me to go crit as well if it's any help to convince you)
Another way would be to click "edit weights", set crit cap for 29%(will be 35% raidbuffed) and hit save. Should tell you to go full mastery after that.