Large Drawers for Understairs Storage

I am currently adding storage under my stairs, the plan is to have 6 large drawers. There are 2 different sizes of drawer (some of these will be angled)
A) 3X 800w*900d*350h

B) 3X 450w*900d*350h

I have framed out under the stairs with 18mm MDF and have purchased 900mm Drawer Runners with 100kg load rating.

My question is how should I build the drawers. My current plan is:

Sides + front/back: 18mm MDF

Drawer bottoms: 12mm MDF

Drawer fronts: 12mm MR MDF

Concerned that:

• Could be too heavy
• 12mm MDF bottom could sag in drawer A
• Unsure of best way to fix MDF bottom to sides - hoping glue and 50mm screws from bottom into the sides will be sufficient.

Last year I made some drawers that were 800 wide and 550 deep with 18mm mdf sides and a 9mm birch plywood base. I rebated the MDF sides and then glued + 18g nailed the plywood base. This is working well, but was a lot of work and I used expensive birch ply bottoms as the drawers would hold clothes/fabric. I am hoping to take some shortcuts this time and just use cheap (unfinished) MDF for the understairs drawers, simple 50mm screws to hold together, and only paint the drawer fronts.

Any tips?