You can wean the tree off weekly waterings by now, as that sounds a bit frequent (at least during times outside of a drought), though that depends on how much water it's being given. If you feel the soil about six inches deep in the root zone and it's somewhat dry to the touch, you can water again, but only once the soil dries to that degree at that depth. Our Watering Trees and Shrubs page provides more guidance.

Southern Magnolias and other evergreen trees and shrubs do routinely shed some of their oldest/lower/inner foliage either in spring or fall. (Older Southern Magnolias seem to do a little all year long, actually, to the irritation of some gardeners, but it's primarily in spring.) Leaves have finite lifespans, and once foliage is too old for the plant to bother keeping (as new growth is shading it more and more as the plant matures), it turns yellow and sheds. Some amount of minor leaf spot infection, which appears to be the case here, is also typical of senescing (aging-out) leaves and isn't of concern.

The branches with entirely-brown foliage and no new growth on the tip are dead, though it's hard to determine the cause. Those can be trimmed back to the trunk (or to a junction with a live branch), being careful not to cut it flush with the bark so the wound seals-over properly.

A photo of the insects in question would help us identify them. Ants can "farm" scale insects, the latter of which feed on tree sap as a plant pest, but ants can also roam a tree for the purposes of hunting other insect prey (which is beneficial), so they aren't necessarily an indication of a problem. If scale insects are present, they'd likely be either Magnolia Scale or the similar Tuliptree Scale. Scale should be confirmed present before considering a treatment for the tree. (There is no treatment for the leaf spot, but it's not needed.) If they are not ants, then we're not certain what they would be without a photo, though Magnolia has few serious insect pests.

The tree doesn't appear to be suffering from a nutrient deficiency, though if you had been fertilizing the lawn, it would get enough supplemental nutrients from that application. The only way to tell if any individual nutrients are deficient is to have a laboratory soil test performed. This will also reveal the soil acidity level (pH) and organic matter content. Magnolias grow best in acidic soil, so while liming the lawn might eventually create a conflict with the tree (lime counteracts acidity), if you haven't been putting down lime every couple of years, soil pH isn't likely an issue. Still, pH level does impact how easily roots can access the nutrients they need, so if it were too high, lowering it would help the Magnolia thrive without necessarily needing to use any fertilizer. We can help to interpret soil test results if you choose to do testing, and it will also inform any lawn fertilization efforts should you decide to boost its vigor by fertilizing yearly.

Miri

You're welcome!