I've been using Airtable[0] as a headless CMS to good effect. I'm enjoying the flexibility of creating and modifying 'tables' using a spreadsheet interface, with the nice bonus of using relationships between tables as well.
The free tier should be more than good enough for simple sites. There's no real concern with lock-in either, since you can just export to JSON/CSV and move on.
You can do forms too. If you're using something like Netlify[1], you can write a function that receives a form request and adds that as a record to Airtable.
And if you're using Jekyll for your site, you just need a few lines of Ruby[1] that lets you pull in content from Airtable.
In the longer run, I'd prefer to use something that uses a similar spreadsheet-like interface, but something that's open source so I can self-host it. But for now, this works.
Note: No affiliation with Airtable or Netlify.
[2] https://community.airtable.com/t/how-to-easily-use-airtable-...