You might collect like perennials that are capable of creating a mound shape, and would tolerate some shade (peony would be one example), and group them into a matched circle or square shape below each tree. It has to be a size that is proportionate to the trees and coordinate with the overall bed ... not too small. This is going to take care of some plants, but there are far too many species so you'll need to find other places for the other things or get rid of them. If the island bed is being used for some moderate screening, the large shrubs are OK, but but I'd separate the green one from the Japanese Maple, moving it toward the back of the bed. As much as possible, limb up the trees, trying to coordinate a uniform bottom (ceiling) to their canopies.

"What is the green plant in the foreground?" I forgot to mention, fill in all the blank space between whatever regrouped perennials and shrubs you were able to use, with groundcover. It could be something that already exists in the plot; you just add more. (I see something low growing at the foreground of the second picture but cannot tell what it is ... if a weed or a good groundcover.) Or it could be a new addition. I don't know what's there, or even where you are, so I'm not calling out a specific groundcover.

Plants have to be moved at the right time and/or by the correct method, or they will suffer. Usually, with perennials, they will get over the suffering and move on.

The stone curb has some pieces that look to be about 10" or 12" in dimension. It would look better if the entire curb had that uniform width. I'd reset the stones, trying to achieve that, if you have access to more of them. It would mean that some of the smaller stones get doubled up. I would also try to set their top surface with some uniformity.

Weeds are always the bane of a bed that has exposed soil. Get rid of them as much as possible, up front. After the planting has concluded, cover all the exposed soil with a layer of plant-based mulch (wood chips, bark, hulls, etc.) Apply Preen according to mfg. directions. Preen is applied on a schedule so be sure to enter the reapply date in your calendar. I'm convinced this is where most people become dissatisfied with it and say it doesn't work. Also, it may not be as good during the first application simply because there will be spots missed. The subsequent application will bring improved results. My Lowe's sells a smallish Preen for almost $20. Home Depot sells a large tub of it (almost 3x the size) for under $30. I would recommend the latter as it is much cheaper per square foot.