Is $3,600 to replace rusted plenum and two blowers a fair price?

My 2400 sq ft home has dual zones controlled by a damper. My Armstrong Air Conditioner, installed in Dec 2020, is not keeping up. It iced over twice last season and is constantly icing over this summer, leading to poor airflow and high indoor temperatures (85 degrees upstairs, 83 degrees downstairs). The unit tests fine, coolant levels are good, no leaks, and output is 10-12 degrees below external temps. The air filter is new, and the external unit is clean. Air flow seems to be the main issue.