I know the A check is 5hrs, but can it get delayed? How close could I schedule it to the next departure? For instance if I have a 6am flight, could I do the A check between 1-6 or would it be better to leave a buffer window after the A check and before the 6am flight?
No, currently maintenance will be always on time.
AFAIK it runs on time so I always put maint checks at earliest dep time after a flight, or have them ending within 10-30min before a flight.