neoforge
sophisticatedstorage-1.21-0.10.44.906

having smooth lighting off makes limited drawer fronts dark
fuctional storage doesn't have the issue (idk what it does differently)

functional storage drawers don't let light through, and don't look bad with smooth lighting on

That's because they use more performance intensive rendering for the items. I only have that for items that require it like chests or in cases where item keeps changing way too often. In those cases the rendering won't be dark without smooth lighting either.

with a block it looks like this

yes that is true, unfortunately alternative to that is that smooth lighting would look worse and light would pass through barrels
as well and because most of the people play with smooth lighting I made a decision to be good with this rendering dark without it and looking much better with.

would it be possible to have a config to move the item model out so it's flush with the front like the block model?
so it doesn't get darkened on the front face

you can convert the barrel to flat face one - just place it in crafting table by itself - and that will make the barrel render without the indent and the item should render without the shadow in that case

that is a satisfactory solution
however there doesn't seem to be a way to learn it ingame