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While leaf yellowing often signals iron or nitrogen deficiencies, the brown dots may indicate a fungus of some type. You can treat the tree with neem oil, often sold as fruit tree spray, to help with this.

This article will have some other reasons for yellowing leaves: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

It sounds like the tree has sooty mold (the black you are seeing), which is actually an indicator of a pest infestation. I would recommend treating the tree with neem oil. This will treat both the pests and the sooty mold fungus. Here is more information that may help: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Most all citrus produce thorns, so this is normal.

I would not recommend moving your grapefruit. It sounds like the tree has borers and this needs to be addressed first. Also, your clay soil should be amended for better growth and drainage. These artcles should help: https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-borer.htm (these can also affect many other fruit trees)
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm

You need to move them to a larger pot, or set them out in the ground. A good potting soil, such as miracle grow can also help in giving them the added fertilizer etc., that they need to come back.

The following articfle should help: https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm

Citrus can be problematic when not watered correctly. Proper drainage is equally important. Too much or too little water will deffinately affect fruit texture and taste.