First, you don't have to feel guilty of anything, since forking open source projects to make changes tailored to your use case is as old as open source itself. It is, in fact, the primary benefit of open source.

Second, it is not a given that your change would be accepted regardless of who wrote it. Maybe the feature is too niche for its complexity, maybe it is better implemented with more generality or extensibility that does not make sense for your own use. In those cases, your change might have been rejected upstream, so having it only locally is a perfect fine solution.

Third, if you believe it is actually useful for broader users, open an issue requesting that feature, and say LLM implemented it in an hour. Then the maintainers can prompt their own LLM to implement it with ease, or do whatever they want with their project.

You could send a comment/open a discussion explain explaining what you did and asking if they would be interested in the feature or a PR.