

Running LanguageTool locally is a bit of a pain, with some manual steps. Plus you have to fetch some data files. You can find around a few projects like this one to make it easier to run LanguageTool.
And yes, as the poster mentioned, LanguageTool keeps some code exclusive to their paid version. There’s a bit of a tension because they ask people not to extend OSS LanguageTool with their paid features.
There’s also this interesting clone, but it seems abandoned.
What volume of data you are discussing? How many physical nodes? Can you give a complete usage example of what you want to achieve?
In general, there’s a steep change in making things distributed properly, and distributed systems are often designed for big and complex situations, so they “can afford” being big and complex too.