He did write some songs with the G.O.A.T.
https://www.rollingstone.com/music/music-news/robert-hunter-grateful-dead-dead-889788/
His best-known collaborator after Garcia, though, was Bob Dylan. Starting with “Silvio,” the two co-wrote many songs on Dylan’s Together Through Life in 2009. “He’s got a way with words and I do too,” Dylan told Rolling Stone. “We both write a different type of song than what passes today for songwriting.” Hunter told RS: “He’s the only guy I work with who I give the liberty to change things. After all, he is who he is.”
Sad to read in that article that he had financial difficulties due to medical expenses, hard for me to fathom.