I would be a little concerned about the low lying areas with clay soil, which is likely to be slow draining and higher pH. Do you know the pH of the soil? Have you tested how quickly the soil drains? You can perform a simple percolation test following the steps in this article:

https://extension.illinois.edu/blogs/good-growing/2022-05-13-improving-soil-drainage

Make sure that the mulch is kept a few inches away from the trunk, to reduce chances of disease or pests. 

For this fast growing tree, the soil should be moist but not wet when checked an inch or two down. Clear away a bit of mulch, insert your finger into the soil down to the knuckle, and it should be dry in the top inch or so before you water, but you don't want it to fully dry out down to your finger tip. In general, trees need 1 inch of water per week, either as rain or supplemental irrigation. However, with clay soil it is best to test the moisture periodically until you get used to your soil and your new trees. I would be surprised if your trees need watering more than a couple times per month given their small size, clay soil, and hearty layer of mulch.