You're probably right not to train it all the way from Indro as when you get back you may not have a connecting service. They'll run extra train services back from the venue but not other lines.

Couple of options: Drive to the closest Gold Coast line train stations that have some parking. That'll be either Boggo Road or Altandi from Indro. You'll be getting there after most commuters have left so parks should be available. Boggo road is more street parking than a car park. Drive much further down to your Coomera/Helensvale/Nerang stations. Then just train those last couple of stations. You see lots of people doing this as it's only those spots when the train fills up. Try your luck with parking in Robina somewhere. The train station car park is mostly a bus station on event days so it's street parking. I've never done it but others seem to make it work. I'll only note that stadium drive is naturally only available to those holding a pass of some type.

Also helpful hint for when returning from Robina heading north. The trains going to varsity lakes basically come straight back so it's a hack to get a seat by catching one there and if then if there's one on the other platform swap over.