The privacy-focused CalyxOS will be available for the Fairphone 4 as well: https://calyxos.org/news/2022/04/01/fairphone4-oneplus8t-oneplus9-test-builds/
LanguageTool is at least partially open source.
You can use it with their server or run it on your own machine.
Their Browser extension used to be open source, but their new one isn’t any more.
https://en.wikipedia.org/wiki/Department_of_Government_Efficiency