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Install a dryer vent termination hood that has no screen, but, instead, has a flapper that swings open when the airflow from the dryer opens it. Do not use one with any spring that holds it closed. Such will create too much resistance to airflow, and airflow is the key to moving the lint up and out.
Most dryer vent termination hoods are designed to be installed thru-the-wall, but there are some designed for roof locations.
Have no screen anywhwere in the duct, since it will collect lint, as you have already seen.
It will also help for you to go up in the attic and insulate the duct for its entire length where it's exposed in the attic. This will keep the dryer exhaust hotter, thereby reducing the condensation of water vapor that dryer exhaust always has. When the vapor condenses due to the cold attic temperatures, it tends to "glue" the lint particles to the sidewall of the duct, and to continue building up.