I use custom categories for my games, both to categorized the style of game and also mark completed ones. You can add to multiple categories.

I also use the hell out of Steam Notes per game, which supports markdown by the way. I use it to note things I've played elsewhere, or unique notes about setting up a game to play, custom settings, or stuff to remember if I don't come back to play for months or something.

They generally sync between my Deck and PC too.