Add a members-only area for any website / create a platform as a service

The purpose of this project is to create a platform from which I can add members-only solutions for multiple websites that can allow adding some code inside their website. I want to create this as a service to offer clients on a monthly subscription. I saw many Wordpress plugins for restricting access and allowing members only to view the content (memberpress, memberful), but I want to create a membership solution for non wordpress sites (HTML sites), that must include restricting specific pages. The website owner must have a dashboard with members and other details, the member must have his own dashboard to manage his subscription, I must have a dashboard with the sites I integrated and their options. I want to offer this to my clients as an online service. A members-only part of a site can be a versatile way of targeting specific content for specific customers. This is done through restricting content access. The purpose is to sell content access to paying customers or a subset of the site visitors (sell memberships to an audience). Some features: - add these options on any website that allows adding code in the site, must work alongside any website, seamlessly adding memberships while integrating easily - create a simple link to a specific page. Once clicked, it opens a payment overlay that looks like a natural extension of the site - customers click the buy link and are shown a simple modal payment overlay - requires customers to log in to access the restricted page(s) - allow access to parts of the site to customers for simply signing up or creating an account and paying a membership fee or subscription for time limited access - manage what part (pages, directories) of the site can be viewed based on the audience (membership level) - the purpose is to restrict access to specific content (this can include tutorials, videos, digital magazines etc.) and allow only paying member to access the pages - it must include options like: customers manage their profile and payment information, recurring billing, account management, transactional emails, protecting content. I am specifically interested in someone who already created membership software and has similar work already done. Starting from zero can mean too much time and too many details for such a platform and we can overlook some small details for membership platforms (integrate payment, create dashboard, add multiple levels, emailing members etc.). I like [login to view URL] platform, so we can use as example. Let me know i you have done this before and how we could do it now. The purpose is to create an online service from this.