While I both understand and empathize with the problem, I think there are multiple ways to avoid this.
1. As you said, you could play ranked. I have seen very few resignations in my ranked games. If you don't want to play them, I get it, but I also think that the best way to get better is to play more.
2. Look at your opponents before joining the game. If there are a bunch of players with 2 or fewer games played, you will certainly see more afks. So just avoid those games, and join the ones that have more experienced players.
3. As Math said, play in a forum organized game. Those who are active on the forums are certainly more likely to be and sty active in their games.
4. Roll with it. While it can be an unfair disadvantage sometimes, it will also be an unfair advantage other times. This happens sometimes, and it is a part of a long term game like this.
And don't forget - those autoresigns often lead to interesting options for diplomacy, too. Makes deals to grab outposts, trade one of yours for one in a better position, send out subs to make it look like you will challenge for it, then gift those subs to the the dormant outpost and see your opponent lose drillers and get nothing. Lots of fun ways to use the situation. Be creative.

"No man chooses evil because it is evil; he only mistakes it for happiness, the good he seeks."
-- Mary Wollstonecraft