I have a different approach than many other players and put a lot of effort into my fullbacks. I target an average defensive rating of 160% and always use a DMC along with 4 DL with the same target rating. No particular preference about which of the five defensive skills is highest. Defense is a group effort so they’ll average across the squad.
About the goalkeeper, I train them actively but their performance is volatile, with even good ones notching some lousy games. But a good goalkeeper having a terrific game can basically win it for you. Goalkeeper performance almost acts like a wildcard - it can win games just by itself, and lose them, too.
Finally - and this is obvious - avoid having the DL get useless offensive special skills. Make sure they get the header and wall special skills instead.
Oh, and speed. Fullbacks need speed in the 130-140% range or they are liable to speedy open field play by the opponents.