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What exactly is the reason for the belt driven Z axis? Stating that you want to keep a simple design and then doing this seems rather contadictory. I don't have personal experience with belt driven systems in this way, but from what I heard it is an absolute pain to calibrate them and (I think) thus the reason E3D has only one big rail.
Otherwise really nice work. I was planning to do something simmilar to what you are doing with a 300x300 base by myself, but didnt force myself to model it out yet