Fully agreed. FOSS maintainers don't owe you anything. You can ask for whatever you want, politely, but accept no or "maybe when I have the spoons, which may be never" as an answer, and don't push.
i'd go with "go fork it yourself". more correct, and less direct. makes it more like a creative insult where the recipient has to think whether they have actually been insulted or not.
I've enjoyed using "Looking forward to your fork/PR!" for a long time now. Caveat: I dont have any big projects nor anything that brings me money or fame. It is possible my slightly snarky responses have limited the projects' potential so consider what you're trying to get out of your open source before following any advice.
I don’t regret open-sourcing this project but god as your project becomes popular bad apples start joining and ruining the party for everyone. Fortunately it’s only a tiny minority, but it’s what stays the most on your mind.