Congrats on getting  the spectacular and heavenly Angel's trumpet to bloom this summer. You did not mention where you garden but I suspect it is a mild winter area so as long as you keep your Brugmansia or Angel's Trumpet cool but not freezing it should slip into dormancy and survive the winter in a cold shed or garage. You are doing the right thing by not watering much as lack of water will help this tropical plant to go dormant.  If the plant fits into your garage you do not need to cut it back. You can even move it indoors as a houseplant for the winter if you have a lot of bright natural sunlight. In spring you can gradually add more water to the pot and allow it feel more sunlight and this transitional stage is hard if you don't have a greenhouse or bright window indoors. Maybe a sunny kitchen window will be the right spot. When all danger of frost is passed move your Angel's trumpet back to the patio and enjoy the summer show. I wait to fertilize overwintered plants until I see signs of new growth. Then I add some Osmocote to the soil for slow release fertilizer all summer but I also give the potted plants a spring hit of liquid plant food such as Miracle Grow as this gives them an instant hit of nutrients to get them growing. Do not fertilize your dormant plants over the winter.