Pilot reported a NMAC on final approach with an aircraft departing opposite direction from a non-towered airport and indicated frequency congestion due to shared CTAF frequency with a nearby airport prevented both pilots from hearing position reports.

Narrative

After we were done with the flight maneuvers we headed back to the airport; the CTAF was very congested/crowded that day; on top of that some people started talking about what type of aircraft each other has on the frequency (it was from another airport in the area that uses the same frequency for CTAF). The wind was calm according to the airport AWOS; so I decided to use Runway 15; managed to find a short period to announce my position about 5 miles to join the downwind. At the same time there was Aircraft X on the ground taxiing to use Runway 33 for takeoff; I announced my downwind; but I did not hear the Aircraft X make a radio call on the ground (most likely due to crowded radio). Because of the amount of communication going on at CTAF; and also because I was trying to give some instructions on configuring the aircraft for landing; I neglected to check the airport to see if there was any movement (the Aircraft X taxiing). I turned base as normal; then final; and when I saw that the Aircraft X was already airborne in climb and heading directly at me about 500 ft away; I turned right and lowered the nose to avoid a collision and performed a go-around. I then heard the first radio call from the Aircraft X after the conversation (by the people from the other airport) stopped; Aircraft X climbed and left the area to NE; we switched to Runway 33 and landed safely; about 30 min later the same Aircraft X returned and landed at the airport; I approached him to hear his side of the story; to my surprise he was very apologetic. He said he only heard my first radio call (5 miles downwind) but not any of my other calls after that; he agreed with me that the CTAF was crowded at that time and it was difficult to make our position calls; he knew I was coming in but since I was in a small aircraft he assumed it would take us a while to get into the pattern and that he did not look at the traffic pattern for me and proceeded to take off to paraphrase our discussion. Fortunately; he also saw me on final as he climbed and gave way to the right at the same time I saw him. Contributing Factors: congested CTAF. complacency of both pilots not to visually check for traffic. Perfect VFR weather (heavy traffic). Corrective actions: each aircraft yielded to the right as soon as the conflict was detected. Visually check for traffic on the runway and in the air. Use CTAF only for necessary communications