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I would recommend having a soil test done. This will help determine what, if anything, is missing in your particular spot so that you can fix it. This should help correct the issue.

You can grow a new tree from the suckers. If you dig around the base of the sucker, check to see if it has its own root system. If it does, you can simply dig it out and cut it away from the parent plant. Then place it in a pot and give it plenty of water until you see new growth. Then it can be planted out in the ground. If you do not see that it has its own root system, scrape a little bark away from below the soil line on the sucker and then cover the wound back up with soil. Check back every month or so until you see roots develop and then follow the instructions above.

This animated guide from the Arbor Day Foundation should be helpful for you. https://www.arborday.org/trees/pruning/

Really, most trees are pruned about the same. Remove any diseased or broken branches. Remove branches that are rubbing against others or are growing too close to others and then trim as you see fit to shape the tree.

These seeds normally require cold treatment prior to planting but once this is achieved, you can plant the seed in some potting mix as with most any plant. Once the little tree is big enough, usually after a whole season, you can plant out to its permanent location. Here is more information:
https://www.gardeningknowhow.com/garden-how-to/seeds/seed-stratification.htm

The first thing that came to my mind was the emerald ash borer - dead branches and leaf loss are possible signs. I would review the following article and verify the presence of this damaging insect:
https://www.gardeningknowhow.com/ornamental/trees/ash/emerald-ash-borer.htm

Yes, covering the roots can suffocate them. But, as it an ash tree, it may be a pest called emerald ash borer. I would have a professional come to look at the tree. This is a very serious pest that needs to be dealt with as soon as possible if it has infected a tree.