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seajaye ,
I have a 520 so I don't know how similar my experience will be with the 520+. That aside, two important things that I have noticed with RWGPS TCX files:
1) The first time you load the course is always significantly slower than the subsequent times, so after transferring the course onto the Garmin (i.e. copy the TCX file into the NewFiles folder), I always open up the course. I also want to change the colour of the course so opening it up that first time is more of a necessity than a chore for me.
2) I find that the Garmin works with the course better if the maps match. For example, I have OSM Cycle maps loaded into my Garmin, so when I build the route in RWGPS I also make sure it is using OSM Cycle maps as I build the route. It is fine to switch to other maps for reference purposes (for example, I find the default Google map shows more landmarks and POIs than OSM), just as long as I switch back to OSM Cycle before plotting additional points along the route.