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You should prune your plants in late spring, after the last frost.
If your needing to prune due to size now, you are likely affecting the blooms for next year.

Here is a link to help you.

https://www.gardeningknowhow.com/ornamental/trees/brugmansia/prune-brugmansia-trees.htm

A morning sun and afternoon shade location is best for your Brugmansia in a warm climate.
They need lots of water to do well, a layer of hardwood mulch can help with moisture retention and keep the roots cool.

Regular fertilizing with a balanced fish emulsion product is important for the health of your plant.
I do want to remind you that a Brugmansia can grow to 6 to 20 feet tall with a width of 10 to 15 feet wide. Also remember that all parts of this plant are poisonous.

The other concern is if your clay soil is holding too much moisture. If the soil is not draining, the plant could be suffering root rot.

https://www.gardeningknowhow.com/ornamental/trees/brugmansia/care-for-brugmansia-outside.htm

Your potted Brugmansia may still be needing more water to be happy.
Check the soil to make sure it is nice a moist each day.
There are some possible diseases or insect issues so I'm listing these links also.
Your plant also may be starting it's winter dormancy as the weather cools and daylight is lessoning.

https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/watering-brugmansia-plants.htm
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/brugmansia-diseases.htm
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/brugmansia-problems.htm
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/brugmansia-winter-care-wintering-brugmansia-in-your-home.htm

This looks like damage from a chewing insect. Chewing insects include mainly caterpillars, weevils, beetles and beetle larvae, and grasshoppers. Weevils are known to feed on Brugmansia, so I think they are the most likely.

http://pss.uvm.edu/pss123/tpbrug.htm
http://ipm.ucanr.edu/PMG/GARDEN/FLOWERS/INVERT/weevil.html

Yellowing leaves on Brugmansia are common. They will naturally shed a few leaves to be replaced by new ones.
If they are dropping many leaves this could be due to high nitrogen in your fertilizer. Cut back on the nitrogen to see if this helps.
If they drop most leaves all at once it could be due to cold temperatures, an infestation or a fungicide treatment of copper.
Here is a link to refresh you on the care requirements.

https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm

Brugmansia plants bloom on new wood, so you can prune at any time; but it will take time for new buds and flowers to form.
Most gardeners do prune the plants in early spring to avoid removing the first flowers.
Brugmansia can take hard pruning and can even be cut back to 1 foot of the ground.
Older branches can be cut back to 1/2 inch above a node.
This will encourage studier branching.
Light pruning can occur throughout the season will help keep the shrub in shape and actually encourage more flowers.