Having some similar issues with my 2018 model 3 RWD LR. When a/c on auto , set on 70F, outside 80F, after 20 or 30 minutes the airflow decreases and interior gets warm. However air feels cool coming out of vent, just reduced flow. Can turn a/c to manual and turn fan to 10 but air flow still reduced.
Then I can turn off a/c and then 30 seconds later airflow returns to normal and is initially cold.
Vehicle is in Tesla shop now and they think the problem is bad actuator vanes and the low voltage controller that supplies them…
(By the way , cabin air filters are new)