In defense of the official ruling, discrete game actions need to be resolved sequentially, so there really is no way to do one thing “when” you do another. One thing must be done either before or after another. That is why I, personally, wouldn’t allow a Shield Master to shield bash before making at least one attack. However, I certainly wouldn’t require them to resolve all of their Attacks before they can shield bash. If you can break up your attacks with movement, you should be able to break them up with shield bash as well.
Dammit, basically every reply I want to either laugh with, or give xp for.
Dammit, basically every reply I want to either laugh with, or give xp for.
Dammit, basically every reply I want to either laugh with, or give xp for.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.