Is decoherence the quantum/classical boundary?

I think they are the same thing. Somewhere around 50,000 bonded atoms will automatically be decohered. Anything smaller requires a decoherence event in its path to be classical.

Can any volume of free particles, coherent light, and 30 micrometer objects (for interaction with particles without causing decoherence) be an environment for quantum weirdness events just waiting for us to do something with?

The most obvious "something" is upgrading quantum computers and harnessing photosynthesis. It's a long shot ..but, maybe even alchemy. Strong, unhindered, wireless power is another candidate. If we find a way to control coherent quantum waves, possibly with the electromagnetic force, we could make hoverboards.