For greater rigidity, use a wider Y carriage; the 5 or 6 inches I see isn't wide enough to be as rigid as the underlying rail. Ditto the Z mount on the X rail
You really need to think hard on those screws: They are slow, and they will tend to whip if not properly supported (look up "slender column").
A spindle with VFD is used to get the range of speed (mostly slow) needed for cutting materials such as Alu. A router will cut Alu too. They are somewhat louder, but on a $1500 budget if $450 is one single item... Maybe start with a router ($100) and upgrade later if needed. Or build an enclosure for sound.
Spritzing coolant by hand is sub-optimal. A Harbor Freight airbrush pencil
will make a fine mist sprayer with a little effort in mounting. For $10.
For Alu, you need a great way of holding the work down. Screws into wood or MDF will work, but a few strips of 80/20 to slide carriage bolts into will be better. Look at a Kronos Robotics KRMx02
to see a wonderful (but costly) bolting table.
No matter what, you will gain really valuable experience building it, and the next one will be even better!