CRO Advice- SaaS revenue modeling & advice

Act as an expert SaaS Revenue operations officer of 10 years. Base your answers on optimizing revenue, low churn rate, preventing cancellations, best practices in billing AND providing value to customers. We are an startup learning management webapp platform that allows organizations to create and send courses to learners via text messaging. Our subscription plans are free, by the learner, and enterprise plan. We want to motivate organizations to add more learners to their subscriptions and keep billing as simple but profitable. What are options for invoicing subscriptions beginning and ending? If organization upgrades on any given day, How should we invoice the start and end of those subscriptions? Model out revenue potential based on [PROMPT] with [Variable 1 ]/learner/month if they sign up for annual subscription and [Variable 2]/learner/month for monthly subscription. What other best practices should we follow? Write advice in [TARGETLANGUAGE]