Honestly, I'd say stop focusing so much on sieges, maybe make it possible but extremely hard to do, and instead introduce some form of wall scaling to allow for thieving. Make it so you get a debuff called "Thief Status" which prevents you from hearthing while on another person's vclaim/pclaim after you scale a wall. Have it also prevent you from being able to destroy/build/kill anything on the claim also to prevent griefing. Introduce offline guarding mechanics like traps or guard dogs. Make it so succeeding in climbing the wall results in an HP hit so you are always at a dissadvantage if you encounter someone inside the claim. Then finally, make it so you can't carry shit while climbing walls, so they can only steal what they can carry in their inventory and have to climb back over the wall to get away with the theft. Introduce locks for houses that can be lock picked with enough time and skill. Adding on to the stress while you are stealing from the place.
Sure it'd probably result in multiple walls to make it more annoying for theft to happen, but there will always be solo people with only a singular wall for you to try and steal from if that is where you get your jollies. It's always possible to come back from someone stealing your items, but having your entire industry destroyed usually means a complete game over.
The idea is that this way, people who are interested in less than nefarious means of acquiring items can go about it without causing "too much" harm to the group they are robbing. The Thief debuff would help prevent griefing but is only acquired when you scale a wall to get into someone's base. It would always come with a risk of being caught (and not to mention all the scents you'd leave behind) but that's where all the planning would come in. If you wanted to increase your odds of successfully stealing, then you'd camp out a little and investigate when people are most likely to be in the base. To prevent too much alt spamming, you can make it so that it takes a fair amount (50+?) of constitution or strength to scale the wall in addition to the skills needed to get the ability. The offline guard mechanics would definitely have to be flushed out more. Would it work that you could build guard dog houses in which a dog will wonder a set "range" and the bigger your domain, the more guard dogs you need? Completely open to ideas. What do people think?