Sanity’s standard plan is free by default, with a pay-as-you-go mechanism for adding extra documents, assets, requests or users. You only get 3 free users, but 10,000 free documents, which may or may not be what you need. In any case Sanity caught my eye because they have a tool for provisioning sites with popular static site generators directly to new Github repositories and can event deploy them to Netlify for you. It can be a nice assist if you want to get up and running with minimal fuss. Even their content editing UI is open source and can be self deployed. This allows for great customization possibilities.