I do this in my app.
I simply start the grid with a ContentNode with a few (~30) sub ContentNode's. On each item focus, I check to see how far they are down the list. When they get (say 75%) down the list I trigger a task that then fetches the next 30 items. As soon as the task returns them, I append those as ContentNode's to the grid's ContentNode. I don't really worry about "unloading" the images as I believe Roku will handle unloading images as they go out of view (and in my experience it will).
Let me know if any of that is unclear. Also hit me up on Roku Dev Slack for more realtime discussion. (link in my sig)
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