Hello,
I have good experiences with Google Apps Script (Google Sheets as back-end for website, JotForm, Mail Merge, transform Excel-VBA to Google Sheets- Apps Script - you can check my profile for this skill)
In my experience, we should come up with a sketch version first (for the first milestone). This first version brings us more valuable insight which help us design a well full-feature version (final version). In some case, the final version is far different from the original design and of course more tasks added. If we start with the full-features version, it may cost us a lot of time and money.
Please feel free to drop me a 'hi' so that we can discuss!
Regards,
NG