Disappointing GPS Accuracy

Today I had my second halfmarathon with my fenix 5s plus. And I am disappointed again. While on my first Course it estimated my distance by nearly 22km instead of 21,095km and today it was around 21,50km instead of 21,095km. This way it is pretty senseless to use the pace while running. What can I change? My first run was GPS only, today I did use GPS and Galileo. My Pixel SmartPhone was like my Fitbit pretty precise. Nonetheless the most expensive device is my Garmin. I really ask what are the 800 EUR good for? Sleep detection is lousy. What is this watch good for? What is the thing which just fits this watch? Hiking?
BTW: The halfmarathons are all officially measured