

Do you know about co-optimus.com? Is that the “third party” you mentioned?
I don’t know of anything better. Setting your filter and sorting by user rating is pretty effective. Aside from that I sort by release date and check back every now and then.
Yes, since most games are simply unrated.
But the “playable” rating does not require full steamdeck support, it just means the game runs. A “verified” rating means a game is a fully seamless experience on the deck.