I know navigating the Desktop Mode without a KB Mouse can be quite frustrating at times, even with the trackpad. You can use your phone as a wireless BT keyboard and mouse connected to the Steam Deck, and navigate the Desktop Mode with ease. I have tested this on my Galaxy S23U (Android) and it works perfectly! This is also convenient if you are outside and want to do some stuffs in Desktop Mode, without having to carry any extra devices.

Google Play Store Link:
https://play.google.com/store/apps/details?id=io.appground.blek

Install the app, run it and connect to the Steam Deck via Bluetooth.