How do I add a Google Assistant command to a button on the homescreen?
Since asking this question, Google has release an app called Action Blocks which does literally exactly what I'm asking for in this question.

With it, you can create "actions" which are either from one of the predefined examples or you can type in arbitrary text which would be parsed as if you typed (or said) it into google assistant directly. These actions can be attached to the homescreen as widgets.

It's not the fastest application in the world, and it's still a bit buggy at time of writing, but it's also only about a week old so I imagine it'll get more solid over time.