Having slabs be placed against the clicked block face and switching with a key really limits how quickly one can build with slabs and doesn't feel intuitive. I was wondering whether you could implement an alternative placement behavior similar to what some other mods have done.

(Sorry could not find a better picture.)

Depending which area of the selection you click, it would place either place against the block (center) or perpendicular to it (sides).

The difference is that one does not have to switch placement modes (taking up a keybind) to place slabs in all configurations.

I don't see how this would be significantly different from current behavior besides having a different visual appearance when placing, I have not used the mod in question but what you are describing sounds similar to the already existing placement modes.

I think I will keep things how they are for now