B2B SaaS Product Billing Software suggestions

We currently have a range of freemium users in our products, and we're now planning to implement a pricing model for our fraud prevention solution. Our current model is based on the number of credits required for 10,000 API calls. For example, let's assume it's 1,000 credits per month for 100 USD, with an additional charge of 0.0001 USD for any overage beyond this limit.

Given this scenario, what would you recommend regarding our billing strategy? In your experience, is it better to build our own billing system, utilize an open-source solution, or explore other available options? How effective are these approaches in managing such a pricing model?