It would be nice if the fenix 5x + can make a difference in default gear between running and trail running. I do both and I have different shoes for trail running and for street running. If set default activity to running for my shoes, they get assigned to both running and trail running. Is it already possible to assign different shoes to trail- and street running? Otherwise, would be a nice feature request.
Gear assignment is handled by Garmin Connect and not by your watch. Anyway, currently it's not possible to assign default gear for activity subtypes, though you're not the first to ask for it.
This would be a nice enhancement to Garmin Connect. I always seem to forget to select my trail run shoes or my treadmill shoes when doing those activities.