Hi Charlie
I think you are not getting a reply on the z plate as most people have it set up in reverse (carriages on Y plate) so can't give you an opinion.
If you want to go with the opposite (carriages on Z plate) then you'll need a calculation of 20mm aluminium with reverse mounting (at full reach and home-they will be different which is why that route us preferred) vs forward mounting with 20mm steel and 30mm aluminium (at any reach as they will all be the same stiffness).
if I get the chance I'll have a look tonight and see if you can recover the stiffness through gauge.
What is the distance from the lowest Z
axis bearing to the collet at full Z extension?