Hi....looks good........I like moving table designs over moving gantry types.
Couple of variations I'd have........mount the Y axis rails on strips on top of the base plate not under it as you show....this makes it simpler to build and cuts down on machining.......also you don't want a swarf trap with the slotted holes etc.
You could also make the X axis saddle longer (higher) to get a longer slide as opposed to a wider one and also make the Z axis slide narrower and longer to give you more support when the Z axis is all the way down......you can go above the X axis crossbeam with the Z axis slide length as it's in the open air.
Before you start scaling the design prior to a build, project the spindle assembly package onto the design and Z axis slide to make sure it won't run out of slide way.
I would also make the base as a separate box with one channel cross member underneath the middle and have the shortened sides bolted to the box sides and have the X axis crossbeam as a rectangular box bolted between the side plates at the top..
By having separate modules bolted together you can align and adjust them in the final assembly and correct any out of squareness you might get..
If you bolt the linear rails onto strips on the base and cross beam you can correct any flatness by careful filing where necessary.....also this will give you height clearance for the ball screw nut under the table and under the X axis saddle.
I would also make the table from a slab of aluminium and have tapped holes instead of the Tee slot extrusions.....just my preference.
BTW....you don't need to machine the linear rail mounting strips....just use the linear rails themselves as straight edges to reveal the high spots and work on them with a flat file etc.....this will cut down any outside machining costs as you can DIY it quite easily.
Ian.