why does the plant become woody so soon and how can this be prevented to produce better foliage?
There are many types of thyme and most benefit from annual pruning. In late fall, cut the plant back to an inch or two. If the plant gets leggy within one growing season, cut it back by half in mid May to mid-June. This will delay bloom by a couple of weeks but will not prevent it. Essentially, you are pinching the stems to encourage a bushier plant. Make sure thyme gets full sun and don't fertilize it. Regular garden soil is fine and too much nitrogen stimulates leggy growth. https://www.gardeningknowhow.com/edible/herbs/thyme/pruning-thyme-plants.htm