Wow Chaz - excellent thread. Great to see someone putting a bit of thought in and going for it!

I would lean towards Thor, the top design, although both would do well.

In my new mk4 design I had to go with moving gantry because I still need to cut large wooden sheets, but with your brief these designs are excellent.

If the granite epoxy is too difficult to make/cast then it looks like it could be done with say 150x50mm sections all welded together. Then backfilled with epoxy granite. Weld on some flat 10mm bar where each rail will go, get it skimmed flat, and there you are.

For the spindle I wonder if you could have a bolt-on high speed spindle for aluminium (router mode), then bolt on a heavily geared down spindle for steel (mill mode). Or have both in place so you can choose.

Good luck with however you choose to develop this - watching with interest.