Do you think most established well known porn stars are actually doing their own messaging on OF? It seems often they are online all the time, like 24/7 which seems unlikely. They have jobs and daily lives like everyone else, I can't imagine they are sitting in front of their laptop the entire day and night. Also a lot of the messaging seems generic language, and very similar between the different stars, which makes me think there are professional OF messengers doing the messaging.

no doubt in my mind that they hire people to message for them, it's probably lucrative af.  
I had a few subs in the past and literally one of them messaged me and the grammar was just off so I responded back calling them a bot and it was such a strange interaction that it was obvious they were almost reading from a script. I would say it's unlikely any message on OF is legit except maybe those just starting off.

Pornstars and top content creators hire managers to handle the messages day to day.  
Given the fact that sexting with a A list pornstar is $50 but a video chat is $1000 !

I've met a couple of ladies who indicated they do not do their OFs.  I only know of one lady who manages her own, but she conceded that if it gets busier, that will change.