I only see one set of leaves that are yellowing and they are older, lower leaves so that is not too concerning. The small brown spots on leaves are just blemishes. Neither is indicative of any disease.
You have it in a very large pot and that increases the probability of inadvertent overwatering no matter how well draining the soil is. All that extra soil retains moisture for a very long time making it harder to calculate just when to water and how much water to provide when you do water.
I suggest that you remove the soil you added to the top of the original rootball - any soil that is not in immediate contact with the roots is extraneous and keeps oxygen from penetrating into the root zone. Then wait until the top inch of remaining soil is dry before adding just enough water so that it reaches that same level of dryness again in about a week. You will have to experiment a bit to determine the right amount.
Keep your Schefflera close to a moderately sunny window.