You can't use 5A on everyone. I actually had some notes somewhere of who it worked on and who it did not, but either I lost them or they became reclassified. I'm not sure which.
You want to use 5A when you can, as it gives you an extra frame to link after the 623C. I would suspect that midscreen 623C 5AC only works on Wala (or anyone else with a hueg hitbox). 2A in the corner works against everyone, though.
As for any hints to learning the timing... there really are not any. You just have to learn the visual cues for it. You do want to delay the cancel from 5C/5B into 623C a touch, to give yourself an extra frame to land it. 623C 2A is a 2-frame link, and 623C 5A is a 3-frame link. Same timing, but you have an extra frame late on 5A since it is a frame faster.
As an aside, you want to do 2C5B 623C whenever possible instead of 2C5C, as it does more damage.