When playing solo it's annoying to have to drop the key to fight, but there's a reason you drop the key when you get hurt, and that's because it's supposed to slow you down or be strategic, such as to clear all the enemies.

A new shield class itself that can break blocks faster than bombs but slower than other weapons (walk back and forth to break the blocks) would make it not just a new bomber shield, but something other classes might want to grab, especially if they have slower weapons.

The boop you recommend helps only bombers or players holding an object. A new shield benefits any class that wants to make them, and adds another fun thing about shield bashing.
I would think these shields would only resist a pure damage type, so normal, shadow, elemental, or piercing as to make them a more "specialized" shield and creating the idea for an offensive shield that isn't just power boosts.
This also makes sure that we don't have to add another button into the game, hurting controller players.

A new mechanic has hard coding in it. Dash and bash didn't just poof into existence. A new shield would be using existing mechanics + 1 new one, and not throw off balance anywhere in the game.

Granted, your suggestion doesn't either. Honestly it's two solutions to the same problem that overlap into different areas. I figure we can add new content into the game that's genuinely useful (shields) rather than just implement a new ability that will only be so useful, and even then only when holding an item. Charging a bomb wouldn't have this power if it's activated by the shield button.

Basically, a shield will prevent this from being used in lockdown/blast network and also make bombers in the clockworks have an easier time with blocks of the sorts (and puzzles that require precision).