Honestly I think the turtling is fine. All of this defense can be beaten by a good offense. Plus offensive power snowballs...as you get more factories you produce more drillers. There are a few things to be learned here:
1. If you initiate an attack on someone ALWAYS finish them off if you can...they will come back to bite you.
2. If you are attacking someone, realize that they may hold a grudge, turtle and do everything in their power to make you lose.
3. There are diminishing returns when attacking someone. Your biggest gains come when you take the outposts nearest to your base. You have to travel less time to get to them, the opponents defenses are less concentrated, and you can reinforce easily. A lot of times your best bet is just taking 3 or 4 outposts from someone even if you totally beat them down. Put them in a position where they can retaliate easily then ignore them. If you spend too much energy trying to finish someone with a ton of subs and specialists turtling then you will hurt yourself on other fronts.