Saul decides to go into hiding in episode Granite State, right after Heisenberg's identity is revealed in the previous episode and Walt goes on the run.
At this point everyone that you listed, apart from Hank, is alive and is a potential threat to Saul. Additionally his association with Jesse and Mike Ehrmantraut is well known to the police. At this point he doesn't know whether the police is able to link them to Walt-Heisenberg. If they are, it would allow them to aquire evidence against Saul that would put him in jail for a long time, regardless how good lawyer he is.
It seems that for him the threat is too big to take any chances and he prefers to go completely low-profile, as he says to Walt when they are in bunker together:
Hey, I'm a civilian. I'm not your lawyer anymore, I'm nobody's lawyer,
the fun's over. From here on out, I'm mister low profile. Just another
douchebag with a job and three pairs of Dockers. If I'm lucky, three
months from now, best case scenario, I'm managing a Cinnabon in Omaha.