Starting to design out my build, looking for critique

[AndyUK]

Is a machine like this even capable of milling Aluminium? Would it be able to cut thin (not more than 10mm) steel sheets?

Others will be able to say, but should be able to take light cuts in Ali. Steel is probably out of the question - for that you're better off converting an existing mill.

The Spindle Plate hanging on the Gantry looks huge to me, compared to the rest of the machine. Did I misdesign this?

Can't see it in the renders? The carriage plates look reasonable - but obviously there isn't a Z axis yet which will change things.

I was planning on using NEMA 34s with Timing belts to drive the screws. Would I need two of them for the Y Axis?

This size of machine should be fine with Nema 23, but that's just a mounting size. On your long axis, you can either go with a pair, or you can use a single motor with a timing belt connecting the screws. If a belt across the CNC isn't a massive inconvenience to you, my vote is for the belt - otherwise dual drive. There is a motor calculations spreadsheet on the forum which will help direct you to which motors to use.

How would you go about connecting together the Profiles? Would angles suffice, or do you use something else there?

You can tap the ends of the profiles and stick large bolts in, you can use angles, or you can use T-nuts. There are probably other ways - look at a Profile supplier's website for inspiration (e.g. KJN).

And finally, is this design even any good? I mostly worked off the information I found in various posts on this forum, especially for the gantry design, but I have no idea if the result has any glancing flaw.

Looks like you're on the right lines to me - I'd carry on by improving the detail of the CAD.