Area gets some am sunshine, maple roots are shallow and soil tends to get dry
Actually, the best thing you can put under a tree is mulch out to the dripline. Other plants compete for moisture and nutrients and planting them can damage the tree's feeder roots. However, here are two articles about planting under trees, and another about planting around tree roots. I hope these help.
https://www.gardeningknowhow.com/ornamental/groundcover/gcgen/ground-covers-under-trees.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/planting-garden-beneath-trees.htm
https://www.gardeningknowhow.com/ornamental/flowers/fgen/gardening-around-tree-roots.htm
The border it is in has ground elder which we are trying to deal with
It is, more than likely, grafted to a completely different maple or sycamore cultivar. This is common, since leaving a Japanese maple to its original rootstock will make it a very slow growing tree.
Unfortunately, your photo did not come through. I will be unable to see the issue at hand.
I would recommend removing the suckers, and keep them removed. Eventually, they will stop coming up, with some persistence.
This article will help:
The tree is between 5 and 6 feet tall, is one of three planted at same time approximately 4 years ago. The other two trees are shorter and unaffected at the present. No other tree in immediate area shows this same bark affliction.
It may be galls as presented in this article:
https://www.gardeningknowhow.com/ornamental/trees/maple/maple-tree-bark-disease.htm
A pot to the border 6 weeks ago and it appears to be still dormant why?
First make sure it is still alive. Scrape off a little of the bark. If you see green, then it is OK. Also the branches should be flexible, not brittle.
It may be that your plant is still trying to adapt to its new surroundings. Make sure it's not getting too much sun.
Here are links to two different articles that should help you determine why it seems dormant:
https://www.gardeningknowhow.com/ornamental/trees/japanese-maple/japanese-maple-wont-leaf-out.htm
https://www.gardeningknowhow.com/ornamental/trees/japanese-maple/japanese-maple-feeding.htm
I would like your opinion based on the photos as to the health of this tree.
Unfortunately, when Maples get this old, they start to deteriorate very quickly. It appears from the photo that this tree is dangerously close to your house. This will cause many problems if not resolved quickly. Their roots are also highly invasive, so if you have a septic tank nearby then I would also have that inspected. It is recommended to plant large maples at least 20 to 50 feet from anything that could be damaged by invasive roots.
Your local extension service will be able to help: https://www.gardeningknowhow.com/extension-search
Watered regularly. Next to a low wall
unfortunately, your photo did not come through.
If the tree gets rainfall at all, then it will not likely need extra water, depending on the type of Maple that it is. Acer is a very broad genus.
This leads me to believe overwatering to be the case.
Make sure to water, only, once the soil is completely dry down to a depth of about 3 or 4 inches.
I would treat with a fungicide, just in case.
These articles will help:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Here is an article that will show you different species of Acer to compare which yours could be:
https://www.gardeningknowhow.com/ornamental/trees/maple/maple-tree-types.htm
It's been in the pot for several of but if I put plenty of correct compost in the soil before moving and plenty of water. Will it be ok
As long as you amend the clay, and it is no longer clay soil, then it can be fine.
Here are some articles that will help:
https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-garden-soil.htm
https://www.gardeningknowhow.com/category/composting/ingredients
https://www.gardeningknowhow.com/composting/basics/ultimate-beginners-guide-composting.htm