I personally have not written anything about that topic, and I’m not really aware of other poems about the topic, but there is a beautiful poem by Rich Smith called “Like Father," in which a man writes a letter to his father and reminisces about old times. It’s not exactly a thank-you note, but you might like it.
You can find it here.
An anon also suggested “The Gift" by Li-Young Lee.
Also, if any of my followers know any poems from a child thanking a parent, please feel free to reply with the poem title or link here so the anon can check the notes and see your suggestions! :)