My X-carve is a smaller custom sized unit. The rails are integrated into the base, the x-axis is solid, I use a custom Z axis with a balls screw and a Makita router. It is driven by a Gecko G540 48volts. It is very, very rigid. I mill a pocket in MDF before the carve to true the wasteboard to the brass work piece. Still, I break the engravings into separate small pieces and re-zero the z-axis before each one for consistency of depth across the entire piece.
The 20 degree should be fine - but more likely to break. 30 degree bits are a little more robust. I use coated and uncoated - whatever is cheapest - I have not noticed a difference in performance. I set my bit to .003" tip diameter and .003" depth per pass (80% stepover) at 40 inches per minute and 30 inches per minute on the plunge. Then, if it is going well, I manually increase the speed during the cut (I use Mach 3 CAM). I also use flood coolant, which is not necessary but extends the life of the bit and provides cleaner pockets.
I prefer to profile the pieces use .25" four flute bits at .01" Doc 40 IPM, if the shape allows. Bits last longer, cuts faster and deeper, etc. Otherwise, .125" two flute at .005" Doc 40 ipm. For holes, I pocket the hole rather than drilling to provide breathing room. I do not profile greater than .25" thick material with a .125" bit.
Hope that helps. Here is an old photo of the base of my smaller, rigid x-carve. The plates have all been swapped to .25" thick aluminum plates with extra wheels.