PS: they appear to be aphids.

Sounds like aphids alright. Insectidal soap is a very effective treatment for aphids and non-toxic. I've used it on my Japanese maples many times to control aphids. Make sure you mix it properly because the soap can damage new growth on JMs if mixed too strong (I found out the hard way). You probably will have to reapply the soap several times at weekly intervals to fully control. Ladybugs are another good organic control measure.

Only our red Jap. maples were affected this year. Tons of aphids dripping sticky stuff. Before you resort to soap or oil, try just spraying them with a firm spray of water. Not enough to knock off the leaves, but more than just a gentle shower. We did several of ours a few days ago and got about 90% of the shiny sticky stuff off the leaves. Stay on it. Those little aphids are sucking the life out of your trees.