Trump attacks Fox news when he can. I believe it is to set up his own news network once he's done. He's got a lot of grifting left in him yet.
No, he attacks certain journalists on FNC, he still watches a crap ton of FNC everyday. Once he's done they'll write him a huge check and he'll either have his own weekly or most likely will just be a contributor that Fox and Friends, Waters World, and the White Power Hour can call whenever they want, just like now but he'll be paid for it.
Starting a network takes too much time and money and wouldn't beat FNC in his lifetime. OANN isn't nearly big or influential enough for him to get into bed with, he wants a bigger reach.