The Edge 500 has a barometric sensor, so you are best off turning OFF the correction.

You will still get some drift, just like your barometer on the wall changes over time. But it is small as your data shows.

You can set the altitude of specific points if you like; do that if you often start/stop rides at the same location (i.e. home, the starting location for club rides, etc).

The USGS data is generally pretty good AT THE SPECIFIC LOCATIONS/BENCHMARKS SURVEYED but those are relatively few and far between by bike riding standards; I'm not so sure that the roadway elevation data is all that good. And all that data needs to be "geo-referenced" into a common reference system... again more errors.

Don't make a big deal out of 30 or 50 feet out of 1000... it just isn't that much.

My $0.02