You are fine for now. You will notice when storage starts to run out. Most important thing right now is understanding how everything works on Linux and the Deck specifically. How it handles shaders, why storage runs out even if you dont have games on the internal storage, etc.

Start looking into som videos about the deck. Download cryo utilities and follow that setup, you learn a lot and speed your Deck up while at it. Only suggestion coming from a fellow 64gb user is to not use more than maybe 8gb of storage for the extra “ram” you can create. You will run out of space. And don’t install any games directly on the Deck whatsoever. Only use that external storage.

When, or if, you start to run out. A 1tb sd instead of your 128gb could be the easiest option and requires 0 tinkering. I swapped the internal storage myself just about 3 days ago and it was super easy. But just getting a 1tb sd is probably best for most people anyway.

Welcome to the gang. Enjoy. Your purchase was a great one.