There are dozens of ways to achive this. While your approach might work for a low amount of users, it propably is too much work to deal with hundrets or even thousands of users.
That’s why I would highly recommend an automated approach, where most of the user handling is done by WordPress.
However, for a low amount of users, I would simply use one of these Plugins:
- Nav Menu Roles
- WP Members
- WP User Manager
Then create a user role like “full-members” or “paid-members”. Let all members register, and give them the same basic user level, when somebody did pay, simply change his user-role from “basic” to “paid-users”. Sure, you need to create a page, where only paid users have access to.
This process is still half-automatic and not scalable to, because you have to edit every user manually.
In case your amount of users is growing and you need to scale your application, I would look out for something like a PayPal-plugin, because you would need a connection to your bank-account in order to check if your money is transferred or not and then trigger a follow-up actions based on the first event.