seems like I'm the only one here that's going to say this but no, I don't ever see a need to upgrade many defenses at all. It's easier to save up for more expensive upgrades if you don't spend the extra loot you have on defenses. I am currently working on an alt and I am at HQ12 with it and I've only built 3 sniper towers and 1 cannon for defense, nothing else. I've leveled those defenses a few times but I've only really focused my resources on offenses. I've got a level 12 gunboat, sculptor level 4, armory level 10, radar level 11 and I'm working on getting my landing crafts maxed. I maybe get raided once a day, sometimes twice a day.

This game isn't like clash of clans where there are a never ending supply of attackers just waiting to hit you because you don't have a shield. If you attack less player bases, you will get attacked less. Prioritize attacking NPC bases and save up player bases for a Dr Terror day. Once Dr Terror comes around, clear as much as you can and hit any player bases you need to finish an expensive upgrade. Once you got your upgrade, rinse and recycle, just keep hitting NPC bases every chance you can until the next Dr Terror day. You'll save up enough resources to get expensive offensive upgrades going and your defenses just don't really matter, honestly.

On my main account I stopped leveling defenses COMPLETELY at HQ 15 and started focusing heavily on offenses. I'm now an HQ 20 with max radar, vault, armory, gunboat, all 8 landing craft, sculptor as well as barrage, tanks medics, warriors, artillery, flare, shock, smoke and zookas, all max. I haven't needed diamonds for an upgrade since I started implementing this method and it's working nicely on my alt as well.