the more hoops you make your route jump through the more likely it is to go all fubar, if you can get used to one using one piece of software or find one that suits your use then stick with that, you say you are using gps visuliser to convert google routes to kml and then to gpx, again many stages for it to go wrong, also i was under the impression that gps visuliser no longer worked unless you had an api key (i may be wrong on that having not used it for a good while) anyway many things for you to think about and try out.
I'm exporting it directly out of Google Maps as a KML file, largely due to the API key issue. Then gps visualiser to convert the KML to GPX. It's arguably too many hoops but at the time I started doing it that way it was largely to avoid using the TomTom MyDrive website as it's shockingly bad (IMHO).
This has worked without issue for the last 3 years, but I agree that any extra hoops in the process are likely to introduce unforseen results so it would be good to just make a route, and sync with the Garmin so I'll give BaseCamp a whirl to map a route that I've done in the past from Home to Bala Lake and then onto Barmouth and see how easy the process is compared to my usual method.
Although Google Maps is easier to map out than MyDrive, it still has it's own quirks. Thanks again Lee, appreciate the input/help!