You don't need bigger pipe, the air goes down a much smaller hose in the end by a lot and you are correct that you do not need multiple regulators. You wont need more air to tools, a little less wouldn't hurt a thing. This is a small compressor and a sander will run it out of wind in a hurry.
1/2 main is ideal for these types of garages. Its way neater with widely available one step bushings and reducers. Copper is okbut I really like black to tell the truth and its the samething as I was told when I was 20,,, its they way to do it and I done it every which way different and am back where I should have started.
1/2 black is so easy to tailor and you can get the fittings tight. I see some exotic setups here and I almost cringe. Not that they wont work and not that some of it isn't a good idea but 99% of home and shop air can way simple served with a couple/few pieces of pipe and half a dozen fittings, a single regulator filter and possibly as little as 1 valve.