not moist enough for root rot, not many pests either, but I know it's been many years since it was pruned last
Unfortunately your photos did not come through. Unfortunately, this does sound like a root rot issue, though.
You can try pruning for shape, but it will not likely correct your issue. In fact, depending on the disease, pruning could make it worse.
You can contact your local extension service for help in sampling for disease or soil nutrient deficiency. This link will help you to find the closest to you.
My beautiful London Plane tree produces many seedlings which grow into saplings very quickly. I usually put these in my green bin which I hate doing but I only have room for one London Plane tree in my small back garden. I would love to donate them but do not know who could use them. Can you help with suggestions, please?
I would visit your local nurseries, garden centers, or any of the universities in your country. There may be a school or local plant place that could use a few extras.
Or, depending on the laws in your area, you could attempt to grow them in containers and sell them!
Here is more information on the tree: