Beyond that, I would create an offering with 1-2 features tuned towards business customers. This kind of tool is a great knowledge management addition, and knowledge sharing is hugely valuable for business customers.
I'd price it the same per seat at the start ($5/m annual vs 9$/m monthly) and sell multiple seats at a time this way. If this works well, you will have found an existence proof of a highly lucrative customer group.
What is a factor is 'getting a deal' and customer conviction. You want to give them an offer that makes them feel good about signing up immediately (increase conversion) and makes them feel like the product has substantial long-term value (induces post-purchase rationalization).
The annual plan with significant sale price accomplishes both goals, with the benefit of smoothing out your revenues and making it much easier to estimate your ROAS.