I am building a website where users can access complex calculators. The key requirements are:

Users must create an account.
To use a calculator, they need to pay a one-time fee.
After payment, the calculator should remain accessible for that user indefinitely.
I explored WordPress and tried several themes and plugins, but none of them work in a way I prefer. Most solutions are designed for subscription models, not one-time payments tied to individual tools. I would like to know how to approach custom development to implement these features.

I looked into membership plugins like MemberPress and Paid Memberships Pro, but they are focused on subscriptions rather than one-time payments for specific tools.
I checked WooCommerce for handling payments but couldn’t figure out how to integrate user-specific access controls for calculators.
I am comfortable using WordPress and some plugins but have no idea how to start with custom coding for this functionality.

I’d like guidance on how to develop this functionality. For instance:

How to structure user accounts to track individual access rights.
What WordPress functions or hooks would help handle payments and grant user-specific access?
Any advice on integrating these features with custom PHP or WordPress plugins.
Additional Context:
I am not from a computer science background but have good working knowledge of WordPress. Detailed explanations or code examples would be incredibly helpful.