In my humble opinion it won't cut aluminium very well because it is not designed to cut aluminium.

Overhangs are your enemy. A good solid base is your friend if you want to get the best out of a fixed gantry.

The tool is unlikely to stick out more than 2" from the chuck, and that's being generous. Try dropping the gantry down to 2" clear of the table and cut the Z axis travel down to 2". See how much more rigid it looks.

If you go vertical and have a good brick wall, consider dumping the frame altogether and Rawbolt everything to the wall. Maybe add extra diagonal braces to stop the gantry flexing sideways.

If you really think you will want to engrave the top of something 130mm thick at some later date, add provision to move the gantry rail up another 80mm. All it takes is a few extra bolt holes. You don't need to worry about extending the Z travel because the tool isn't going to be 130mm long.