I've just made a self-hosted CORS-enabled waitlist that writes to Google Sheets for myself.
It's the most economical option. Google Sheets is free. One instance can serve many many landing pages. The landing pages wouldn't need backends and can be hosted for free e.g. Netlify, Github Pages.
The integration is also seamless. It utilizes CORS. You can make a waitlist form on your own HTML and just do a cross-domain AJAX request. This gives the highest flexibility in terms of styling and customize the after actions.
The alternatives often use iframes, which are more difficult to style and customize because you won't have direct access to the content within the iframes.
I use it to power >10 of my landing pages, which are all hosted for free on Netlify. The Wait server is hosted on OVHcloud for $4/month.
I'm looking for early users who are interested. If you are interested, please let me know.
The repo: https://github.com/tanin47/wait
Thank you!