I've never seen a setup like you describe, where the CNC parts hang from the ceiling until you want to use them. It sounds like you'd have problems getting everything to work right each time you did that. Maybe if there were solid bolted connections that were repeatable you could make it work, but it seems that if I had a rig like that I'd probably tend to leave it in place rather than go through all the trouble of re-connecting everything each time.
It also seems that for the sort of projects you're talking about, conventional woodworking equipment like tablesaws, bandsaws and chisels, etc. would be faster and work better than CNC. Remember that with a router table, you can only work on one side at a time, but for putting tenons on beams you want them positioned end-up. With fancy engraving and carving, you're more in the realm of where CNC router save you time.
All the GRBL setups I've seen have been low-powered, suitable for smaller machines perhaps, but nothing as large as the one you're proposing.
Take a look at DeskProto for simple, powerful CAM intended for non-machinists.
Lots of machines use ball screws over an 8 foot span, but they tend to be fat ones. However, rack and pinion is a viable alternative.
The HGR type of rails, with a basically square shape, are vastly superior to the round rail type.
Only the most solid CNC routers can do an effective job on aluminum, and none of them cut steel well.
Laser cutters are entirely different machines that only superficially resemble CNC routers. Most importantly, they are totally enclosed with eye-safe material and won't fire until the doors are closed and it's safe to do so. I know there are videos of people who strap a laser onto their CNC machines, but people do lots of stupid stuff on the internet.