hi jase,im not clued up on the 623 but the A6 is in my eyes cr*p.i drive a truck up and down there its traffic light after traffic light potholes and speed cameras but there aint much other choice really it can get narrow in parts as well you wont be rushing down there.
what i would advise is to avoid queensferry as they are building a new bypass which wont be done by maytime i was down there 2day i would suggest getting onto the m53(wrexham/chester)then the A55 towards wales its a few more miles but generally flows freely if its bank holiday i would certainly go this way