The best alternatives to Darklang are Treeline, FieldGoal, and Syncano. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of Darklang?
Notion— Get over $6,000 worth of Notion + unlimited AI, for free
Treeline makes it easy to build custom, high-performance apps for Node.js. It is designed for building practical, enterprise-grade backends in a matter of hours - not months.
WebSubmit allows you to build an API for your web forms, manage data submitted from the users, secure the forms with modern anti-robot mechanisms, trigger automatic emails, auto-delete data etc,. Headless CMS + WebSubmit as backend would be a perfect fit.