One of these days some bright programmer is going to make a GPS program that will.
1. Import a KML or GPX file.
2. Break a track with >500 trackpoints into multiple segments, each with 500 or fewer trackpoints.
3. Send it to the GPS
Some smart GPS company is going to make a GPS that will import tracks of at least 100k trackpoints.
Good lord... I've had to download 3 programs to do the above and it's still a nightmare.