I prefer a center brace myself on that span, and it gives something for your doors to land on, no light shines through, and it's solid.
This first pic is how I've been doing for many years now, and my center brace is actually removable w/ the screws on face, just in case.
Of course there are verticals behind each 2x4 that give support directly under the rim joist/top horizontal.
This next pic was when I first started building stands and before I made those corrections, but it shows the top flat 2x4, and this I think really helps flatten out any minor deviations, especially w/ ply top.
I use wood glue and screws always, the ply subtop helps stitch all together and adds a great deal of shear strength, keeps from racking.
This may be overkill on weight load factor, but it makes easier for perfect coplanar top especially for novices and also deals w/ any minor crowns.
I just prefer the flat and also the mirror of top and bottom makes easy to square up perfectly.
I always measure all diagonal measurements as I go to make sure square before hard fastening. I usually pin nail it together before I screw, so I can push it square if my diagonal measurements read needing that, the pins bend, screws secure all exactly where I want it to stay.