https://valheim.fandom.com/wiki/Building_Stability

You'll need to use iron supports if you want stone walls higher then otherwise possible, but eventually you will reach a point where you can't get higher, and then all you can do is build wood on top of the stone (the first piece of wood should be blue) and continue on with that.

Another option is to raise the ground in select places, as high as possible. Trim down the edges if needed and hide the entire thing with walls, making it appear as some kind of thick pillar. Then from these ground pillars, you can build fresh iron supports to carry nearby stone walls even higher.