While i agree about the weight, after all i managed to make 1000x400x200 machine to weight 200kg, something that i don't see on this and many designs are the ribs. Apart from the careful listening to advice and using quality components, the machine in question is build so that there is not a single part of it that is not insured against bend in any directions. By ribs. Hence the precision achieved.
Also there is not a single detail which gives doubt about weakness. Where it looked weak on the initial design, i designed it so that the weak places were reinforced by smooth transition and angles made so to transmit forces in the direction i liked.


Point that look not right to me on your design:

Ok, you will weld 2 beams together for the gantry, i did the same , but where are the side plates that are soldered to them at both sides. They will bend and if not, they will vibrate. They can not be left open.


Gantry legs. i see one plate in the middle that will reinforce them . One is enough against left right bend, but not against twist. i would put 2 on each leg so they form a box.


Web Goblin, your gantry looks very nice. What you intend to do with the gantry legs? Why you did them like that? You plan to fit the screw there? I contemplated this design for compactness, but for my liking it seems weak for left right forces. I would have reinforced this place with closing the beams and drilling holes from above to fit the screws to the bearing blocks.