I’ve been looking around and sadly haven’t found any solutions for it myself, although in case you didn’t know you can actually self-host LanguageTool and keep everything on your own hardware, it even simplifies the process to connect to it if it’s self-hosted on the same device it’s used on. Seems like an okay-ish solution that I’ve been using for now, though it still locks you to only their free features, but it does work without internet access. Hopefully someone can come by with more alternatives to it as well.
Edit: Also LanguageTool is actually open source itself if you’re mainly worried about the source code thing.