Just in case any member are interested there was a major update recently V2.6.1 for IOS TomTom Go App which added an important missing link. You can now save routes made in the app to MyRoutes. Also routes created in MyDrive can now be synced to the App. For those how prefer a different planning platform the desktop version of MyDrive can import GPX files, which then can be synced to the App.
This now makes the TomTom Go App a creditable alternative to dedicated SatNavs. However it's still not possible to load a GPX file directly into the App and the import function in MyDrive is only available on the desktop version.
But at least now routes can be created and saved. It's been a long time coming.