If I had to sum mine up it’s been Isaac on the Steam Deck and Tabletop Simulator on Linux. And a bunch of other stuff on both.

My boring advice is to spend a few weeks using it as it is out of the box: just play your favorite games on it.
After a while you might get a sense of what you would like to change (if anything) and then you can research how to go about it.

For casual games on the Deck some my most played are The Binding of Isaac (most hours by far), Brotato, Deep Rock Survivor, Slay the Spire, Balatro, Dead Cells, Crypt of the Necrodancer, Shotgun King. But there are so many good choices, it really depends on your taste.

Looks great! I used Twire in the past but this is a lot more polished and doesn’t require using an account.