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I would recommend treating the plant with neem oil. It is safe and effective. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

You should go ahead and do it now. Normally, you would cut them back after repotting anyway. If you want to repot the plant, go up about 2 inches in diameter, or another couple sizes. Regular (well-draining) potting soil should be sufficient for your brug. Give it some slow release fertilizer. There is conflicting information on its NPK, but using a ratio of 6-10-4 or similar is normally adequate. This article should help with its overall care: https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm

Use liquid fertilizer on your brugmansia 2 -3 times a week. There is conflicting information on its NPK, but using a ratio of 6-10-4 or similar is normally adequate. Spider mites and aphids are among the most common pests of this plant--both of which can be taken care of by treating the plant with neem oil. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm

Did you get this plant in the last few months? If so, it doesn't need fertilizer. Yellowing and dropping leaves often indicate soil too dry, but if you're sure the soil is damp down where the roots are, that's not the problem. Brugmansia are not really houseplants, so to keep them indoors you have to put them in a very sunny spot. Too little light could cause the leaves to fall off. But if it's getting plenty of light, there's one other possibility, and that's spider mites. Here are two articles, one on keeping brugmansia in containers, and one on spider mites.
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm

Your plant dies back, or you cut it back, to overwinter, is that right? Zone 8 is the northernmost area, so a hard winter, like we all had last year, could well affect the plant negatively. Perhaps it just didn't have enough energy to do the flowering thing. It should recover next year, if the winter is not too hard. Continue to make sure it has plenty of water; try adding more phosphorous in the form of bone meal to your fertilizer schedule; and maybe use more mulch this winter. This article has more information: https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm

These plants bloom on new wood. Cutting them down that far inhibits blooming. Generally, the best time for pruning is spring since blooms usually begin appearing within a month or so following pruning. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/trees/brugmansia/prune-brugmansia-trees.htm