Rob Chapman's biography is very thorough - and hard on Pink Floyd after Syd was forced out. To my mind, it tells as though his mental health problems started in earnest after he was sacked, though these may have been underlying after his dad died.
The way Chapman tells it, it seems that painting was the musician's first real love. His turning his back on the music business is put down to him not being capable of dealing with the promotional aspects and being too emotional to deal with the more hard-nosed business. He isn't made out to be a saint - his cruel treatment of then girlfriends Lindsay Corner and Gayla Pinion needed the man to have been given a good talking to, regardless of how cool his friends wanted to be seen as at the time.
When Robyn Hitchcock (ex-Soft Boys) talks about how he was influenced by Barrett musically, he realised in his 30s that he didn't need to suffer what the man had to be successful in an artistic sense.