You don’t need to mark as an asset. You just put into a collection(M) or put into a unlinked collection(Ctrl+G then F9 to change the name). Then you can press shift+a->Instance collection->Then click the collection name.
There’s another type of instancing and it’s used with the parent child relationship. For example here, the cube is the child of the circle and if I select the ring I can instance it’s child on every vertex: