architect and build a web app customer portal (payment gateway , billing, crm)

We provide local businesses an advertising platform to advertise locally in our partner locations such as retail stores. We require long term freelancer to develop UI, easy to use, self service customer portal • purchasing subscriptions and products (sign up wizard including payment) • customer can check which retail store to display ad. • managing subscriptions including uploads Ad • managing customer details, • managing customer financial details, payment details, invoicing • email reminders when subscription expiring No wordpress, paypal please - must have experience with architecting, developing customer portals, billing, payment gateway integration, CRMs and worked with cloud workloads. Must understand security best practices. Will we consider any SAAS integration offerings, authentication, database etc. Important points: 1. Must be easy to use for customer 2. Architecture must be secure, scalable, robust Current state: 1. Our website is in a very basic form, hosted over Webflow CMS [login to view URL] and we use jquery to launch payment forms and php to process payment api 3. Our payment gateway solution is by Mastercard Simplify hosted by Commbank Australia ([login to view URL]) 1. Please help us understand reach out to us so we can discuss, whiteboard diagram or roughly explain on how solution will come together. 2. Please share the examples of work. thanks