Not bad at all for a first attempt!!!
You didn't mention the machines size which is always a factor. In that regard im somewhat concerned about the gantry construction. You may need larger beam extrusions depending upon the span and your expectations. Also the uprights supporting the gantry beams might need better support or bracing.
As for avoiding drilling and tapping you still do a lot of that with extrusions especially the tapping. Of course if you buy a kit that would be done for you. Also to achieve your ladder frame you will need to drill a lot of holes for access to screw heads in the T-slotted extrusions or go with alternative mounting methods. You really don't want to rely upon just the gusset blocks.
You might want to consider that mounting two gantry beams in parallel will make life more difficult for you. It will be more difficult to get the beams in the same plane and parallel to each other thus making it more difficult to mount your linear bearings properly. How much you need this approach depends a lot on how much Z you really need.
In any event mentions of probable usage and work space size is a big help.