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, 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 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.
ProcessWire is a Content Management System, not a CMS which includes a design or layout system like Wordpress.
So as a developer you are free to develop the front-end the way you like. Which is easy with the provided API.
I use it for API's, small and huge websites. Love it.
The tendency is to have very light front-end, while consuming the content from some services. At the same time, content clouds and headless CMSes haven't really taken off yet.
They are all managing content, but in different ways
If you're using something like React or Vue, then I'm guessing any CMS used as a headless API would probably be ideal, since the system would only be used for creating content/the admin area.
Why risk your app on dependency of some CMS?
If you are looking for market in extending some popular CMS, then you probably should go for some more popular CMS, which currently is WordPress. Unless you are in an industry which has some nice CMS usage like Drupal or Joomla.