After a lot of thought I think I have thought of a way CA can address sieges and settlement battles, that would be both enjoyable, and allow CA to utilize assets and systems they designed for Warhammer 3. I will break this into 3 parts to address 3 main points, the supply points, settlement defenses, and garrisons.
So first off, supply points are cool, but I think the best way for them to be implemented would be to have them be a set fixed resource that is spent during the deployment phase. The current implementation is a cool concept but doesn't really fit how total war plays. But supply could easily be used to set up deployables before battle like previous total war games had.
Second off, I find that settlements on the campaign map should have a building chain that only impacts your garrison, and your actual settlement defenses should be sequestered into its own tab, or mechanic where you pick and choose which settlements get a certain level of defensive structure that is separate from your settlement building chains. This could be paid for with gold or some other unique resource, but it would give the player the option to pick and choose how/where they want to defend their empire, and not be limited by being linked to settlement level or a building chain but rather what a player is willing to invest with their available resources.
And finally, thirdly garrisons are in a strange place right now. Some are strong, some are weak, some benefited from the changes to siege/settlement battles in that last patch and some suffered, I think garrisons should not be a pre set group of units, but the player should be able to pick what units are in their garrison. This would be tied to the above mentioned garrison building chain in your settlements. This garrison building as it is upgraded would unlock more slots for units, and more types of units for you to choose from. This building can be different for minor settlements and major settlements. Minor settlements having smaller weaker garrisons and being able to upgrade the building to tier 3. And major settlements giving you access to more powerful garrisons, and the building being able to be upgraded to tier 5.
I hope these changes sound agreeable to people. I have no idea how difficult or how long they would take to implement but I feel like they would put sieges and settlements in their best playable form for the Warhammer trilogy. At the very least hopefully give CA some idea on how to address this topic.