You're not going to find any DIY routers accurate to .001".

You mention repeatability, but also talk about flatness, which is something different entirely.

If you need .001 accuracy, you need to weld a frame, have it stress relieved and machined for rail mounting surfaces.

You'll also need expensive screws, as the cheap ones used by most are only accurate to ±.003" or so per ft.

Another option is to weld up a frame, and use self leveling epoxy to get flat rail mounting surfaces. That gets you a coplaner surface, but you still need a way to get everything mounted straight.