Tree care - am I hurting my trees?

First off, if I'm in the wrong sub please tell me where to go.

Backstory: four-year-old house with street trees (3 of them) planted by whatever landscaper the builder used. They appear to be Sycamores but I've been told they could be London Plane. I don't remember how tall they were when I bought the house but they're roughly 20-25 feet now. They've had a couple of good years and a couple of "could have been better" years. I wonder if I'm the problem.

Landscapers love volcano mulching. I undid that as soon as we moved in. I removed most of the excess mulch but left a flat bed of mulch around each tree, probably about 3' in diameter. If I had my way, I'd remove all the mulch and let the grass grow right up to the trunk, like in real life. But, in deference to the neighborhood "norms" and my wife's desires, I've left the flat mulch bed.

Here's what I've been doing that may (or may not?) be harming the trees: the grass in the lawn is a blend which includes a creeping variety that spreads underground via rhizomes. They grow into the mulch so once or twice a year I use a manual edger to "refresh" the edge of the mulch bed which also cuts off the rhizomes and allows me to more easily pull the unwanted grass out of the mulch bed. The edger cuts about 4.5" deep.

Am I also cutting off tree roots near the surface of the ground? Could that be the explanation (or even a partial explanation) for why a couple of years have been so-so for them?