Freemium or nah?

Yeah I've found 30 day trials work for me too!

After working with a tonne of SaaS companies, I'd say 15-30 day trial periods are better.

For your product, I'd say 30 days then near the end of the trial period send users an email/notification along the lines of;

"ChurnKey could cost you nothing and recover XXX in revenue for you every month because this month we already recovered XXX revenue."

Or something similar, I'm sure you get the gist.