Hi,

from all what you say and having in mind you have steel top and 2000x1000x15 aluminum plate at hand, if i were you i would start again with some parts of the drawing.

For example make gantry back plate one whole piece, not 2 pieces that will bend like crazy, use the stiffness calculator found here in the forum to confirm and strengthen further if needed. Same with Z. Maybe read #8 here where i explain much better Z design with the same effort

Generally speaking better look all the similar builds as there are much better designed machines from aluminum plates and similar rails.

About the rails, i assume you know already and they told you - square rails are the way to go.