Ok, so this is a surprise. I moved. About 400km to the east, a bit to the south. In my old town I'd get differences between tracks recorded by phone and Fenix of up to 300m over 5km, and pace differences as much as 0:30 slower on the Garmin. I've lived there for quite a long time. I optimized my running path to avoid as much higher (more than 2 floors) buildings and trees. Let the gps soak for quite a while. No such luck. Sometimes the deviation wasn't too bad, sometimes it was super bad. I always was massively short on measured race routes on the watch while my phone was pretty much spot-on.
So now I moved. Now I'm getting about 10m difference on the first km, minor differences on every successive km. Yesterday I had one of 30m over 4km and a pace difference of 0.06min/km. I've not avoided trees, satellite coverage is as good/rubbish as at the last place. The other day I ran a path in the dark, totally shaded by tress on both sides and the deviation between both devices was still minimal. Yes, there are still those odd drops in pace in my Garmin run, but they are much less common, and less extreme. No firmware update. I'm still running the same fairly stable beta. Basically I'm confused.
I wonder if there's an external factor to which the watch is sensitive to. Transmitters that mess with receiving a good signal? Something else? Fact is: even my handheld gps was never really good in town. The place I used to live in is full of tv and radio stations, so maybe something related to that?