I am also a bit of a gadget junkie and with my Steam Deck set for February fulfillment, I have been doing the same thing as you. Commonly, anti-glare protectors are labeled as "matte" if you were wanting to be googling around for it. In my experience at least, if it is not a popular smartphone or tablet, it can be more difficult and costly to get a good quality matte screen protector. Two base requirements you should always have if you do get one is that (1) it is glass and not plastic to maximize display clarity and (2) it is 9H hardness so it can adequately resist scuffs and scratches. With those requirements in mind, I only see a single matte glass screen protector for Valve Steam Deck available for sale online. The upside is it is manufactured by the company "dipos" in Germany which means it come with the kind of world-class quality craftsmanship you would naturally expect from German-made goods.
EDITED: Found a cheaper, unlisted purchase link by searching eBay.de and plugging that the product ID into eBay.com. For some reason, it isn't shown in the eBay.com search results when I search for "deck dipos." Only 0.99 Euros international shipping so this is an incredible bargain!
Find many great new & used options and get the best deals for 2x Screen Protector for Valve Steam Deck matte Flexible Glass 9H dipos at the best online prices at eBay! Free shipping for many products!
www.ebay.com