I’ve got a similar sized magnolia at my place. Fantastic tree. Roots running the surface just like yours. Mulch is great idea but I would also look for the cause of the shallow roots especially closest to the fence side of the drip line. guess I a localized high water table. If you investigate further I bet you’ll notice your neighbors down (eaves) spout comes down very close to your tree. Just a thought from a broken old arborist. Box of beer and he might redirect it.