Grubs are not unusual in lawns, but grub damage is actually pretty rare in lawns. Many other things can cause dieback in lawns.
It's normal to find a few grubs when turning over soil. (In fact, it's good to have a few--because it keeps grub predators and diseases around.) Read about the threshold at which grubs can be a problem: https://extension.umd.edu/hgic/grubs-lawns This page also includes a link to controls if that is necessary.
Unless you have a true grub problem, don't apply grub control. It will be a waste of money. More importantly, it does not only affect grubs. It is toxic to many organisms in your soil's ecosystem which you do not want to unbalance.
Fall is the best time to overseed and fertilize. Mid-October is the recommended cut-off for overseeding.
Here is a very helpful and complete guide to successful lawn management: https://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG102%20L...