I'm very unsure as to whether DnD Beyond is worth me buying everything twice, and it's a hefty intials investment.

I'm sure myself and others would be more willing if we'd been given the option to try it out a bit before we buy.

You can access all of the SRD/Basic Rules content, and can make any homebrew (including homebrewing content that isn't free, such as a subclass that you own in the form of the PHB) you want for free. 

A trial, for a model they don't use? Yeah that's a great idea, wouldn't be confusing at all.

You already have the ability to try the platform out, without time restrictions, just with restricted content, as the poster above says.