I think he sounds so content in all these letters because getting away from all that shit is actually good for his mental state. Chris's pursuit of short term happiness leads to long term unhappiness. In jail, many of his worst impulses like being online 24/7 are restrained. He's reading books, which is infinitely better for your mental state than reading social media. He gets three government-approved meals a day that he can supplement with garbage, as opposed to his diet on the outside which was 100% garbage. He has regular, structured interaction with other humans, something he hasn't had since high school besides apparently raping his mom every third day. His living spaces are clean, at least by Chandler standards, something he probably he probably hasn't experienced since he and his father lived separate from his mother in high school.
I can totally understand how Chris could be happy in jail. The life he's made for himself on the outside is so fucked up and sad. A more regimented existence isn't going to fix Chris (what could?) but maybe like dogs and children, autistic mommytouchers crave structure deep down.