Senior WordPress/PHP Developer needed, Custom Enrollment Portal with DocuSign & Stripe

We're a marketing agency building an enrollment portal for a client's B2B program, launching September 1. This is a custom development project on WordPress — not theme customization or page-builder work. The build: a standalone portal (subdomain, managed hosting) where invited companies enter an access code, see their company data pre-filled from a ~2,500-record database, review a program page, then either enroll or request a meeting. Enrollment includes a scroll-enforced terms acceptance and an embedded DocuSign signing ceremony generated from a pre-filled template. Two paid services are sold via Stripe Checkout. Admins manage codes and region eligibility through the WP dashboard with CSV import/export. Designs, copy, and PM are handled on our side — you build. Must-have skills: custom plugin development (hooks, custom tables, WP REST API); third-party REST API integration from PHP, including OAuth/JWT auth and webhook handling — DocuSign eSignature API experience is a big plus; Stripe Checkout; solid vanilla JS for multi-step pre-filled forms; security fundamentals (nonces, sanitization, server-side lookups, hardening a site that handles legal documents and payments). Scope & timeline: ~100–130 hours over 3 weeks, starting immediately. Core flow demo-ready by Aug 21, launch Aug 31. Fixed milestones or hourly — tell us your preference and rate. To apply, briefly answer: (1) Describe a custom plugin you wrote and why an existing plugin couldn't do the job. (2) Which OAuth/JWT-authenticated APIs have you integrated from PHP, and how did you handle token refresh and webhook verification? (3) How would you store and query a 2,500-row lookup table in WordPress without exposing the data client-side? Applications without the three answers won't be considered.