This isn't a secret by any means.

Even Google Chrome's built in Spell Checker is constantly sending data back to Google and functions as a keylogger.

Do you mean the enhanced spell checker which is off by default? The regular spell checker is done locally.

https://www.google.com/chrome/privacy/whitepaper.html

LanguageTool Plus [0] is an open source (optionally self hosted) replacement for Grammarly (a paid version is available but the free one already finds more mistakes than the free version of Grammarly did for me). Available for FF and that google browser ;)

[0]: https://languagetoolplus.com/