I would also suggest that you consider that the two sets of 135x12 and 2 sets of 225x12 are basically an extended warm up - considering you are able to perform 315x12 for two sets.
You may want to consider 135x12, 225x12, and 4 sets of 315x12 as an alternative protocol.
I train traps immediately after rack pulls... so when I finish my heaviest sets of rack pulls, I drop off some 45s and start shrugs and typically perform 6 or so heavy sets of ~8 reps.
As I am transitioning from rack pulls to shrugs, I don't do any warmups.
Some good alternative exercises here, gonna try some of the calf machine shrugs (strained bicep tendon needs some rest)...