I have been working on Wait: https://github.com/tanin47/wait, a self-hostable CORS-enabled headless waitlist system that connects to Google Sheets.
I have many landing pages hosted for free on Netlify and Github Pages as static pages. All of them have waitlist forms that send cross-domain AJAX requests to the Wait server, which then writes the emails to Google Sheets. Since there's no iframe, it's easier for me to style the form and customize the after actions.
The Wait server is hosted on OVHCloud for $4/month. It's probably the most economical option for a waitlist system.