We read your developer brief, the Apps Script code (about 750 lines), the landing page, the test guide and the setup guide. The prototype is well organised and matches the brief closely, so we are comfortable taking it to production.
The brief says the prototype has been checked with simulated data but not run end to end in a live Google account. We have read the code and have not run it. Our first step is a full live run in your test account. If that turns up work beyond this proposal, we tell you before spending the time.
Our first guess in chat was 30 to 40 hours. After reading Section 9 the job is bigger than hardening: 14 launch-blocking items, two installs, browser testing, a handover call and 30 days of support. The estimate also includes our QA engineer's independent testing, which you asked for. That is why it is now 80 to 100 hours across the team.