There aren't many large-screen ereaders.
But if the DX works as-is, does it really need updates? Updates for what, the sake of being updated? Many people use it and enjoy it. They don't need Kindle FreeTime or Cloud Collections, and AZW3 compatibility is not a dealbreaker -- you can still use MOBI. Many people use MOBI anyway even for new devices.
Don't reject the non-updated DX until you have ascertained that as it is it needs updates to be usable for your dad.
Anyway, good luck in your search.
