This seems incredibly out of touch; it seems like you have not witnessed a raid in a while. First of all, settlements without walls with or without a guard force are utterly dead pretty much if they get raided. Even settlements with a guard force and with walls are easy pickings for raiders; you said the defenders have the advantage? The defenders do not have the advantage, and this is why.
A- The Raider chooses when and where to raid. You can easily see how big of an advantage this alone gives.
B- They know that they are raiding so they come fully prepared, a guard force is utterly unprepared 90% of the time no guardsmen (Is walking around a city wearing prot II armor, OP weapons and having a invbar of potions such as strength, etc. Most guard forces are not capable of such a thing anyway.
C- WALLS ARE NOW UTTERLY USELESS. I do not know if you know this or not a guy with a grappling hook can now literally pull a full army up any walls.
D- The Raiders shall have the better gear, most likely all the time - Raiders tend to be a faction based mostly on PVP, they grind skills or have grinding alts to make them weapons and armor, potions extra. While a guard force mostly Roleplays and does not put too much effort into the "GRIND."
E- This only creates more factions based solely on the grind and the PVP. Which is shite, PVP has a place, and I like PVP I have supported PVP for most of my time on the server. BUT PVP can never be a focus PVP is an outcome that comes out of Roleplay. Not the main sought after thing!
Now in conclusion, the real issue, the real problem is not PVP nor is it raiding the real problem is (THE PVP PLUGINS) grinding the weapons, the armors the potions. That is the issue that creates PVP focused factions, I do not think any current PVP focused factions enjoys the grind, I bet they want to focus more on roleplay, but they cant with the current system they are forced to grind skills or Macro Craft.
To fix to raiding and to an extent warclaims, you as a "War Team" need to ask the plugin team to fix this broken system of grinding and crafting, OP potions, op armor and op weapons this all needs to rebalanced and some removed.
And get rid of grappling hooks < Its worse than cannons for walls.