TrueSteamAchievements has this (in Easy Achievements). Source: 6 months ago

It does happen perfected games get new achievements added. Maybe that's what happened to your missing game? It's a real drag to make a separate list to compare Steam's list to, but maybe it's worthwhile signing up for something like True Steam Achievements if it's important to you. Source: over 1 year ago

OK looks like they've been picked up by truesteamachievements.com and some people have them unlocked. Source: over 1 year ago

A other way would also be to go on: https://truesteamachievements.com/. Source: over 1 year ago

Whether 100%'ing a specific game is worth it or not depends entirely on your preferences. I wouldn't put too much focus on achievements unless it brings you joy. I can't give you recommendations since I don't know your tastes. I'd simply pick a game you like. This website can help you track your progress, and it also has guides for most achievements. Source: almost 2 years ago

Howlongtobeat.com and then filter by your desired length and sort by rating or popularity. Source: 6 months ago

Overall, I would say that Mario Wonder was good, for those reasons. However, that's about all the game has to offer. For starters, the scope of the game is very small. Especially considering the $60 price tag. The game only me around 10 hours to beat, or about 18.5 hours to 100% according to howlongtobeat.com. Source: 6 months ago

Ive got a big backlog of games to play, so what I did is for the singleplayer stuff I looked up everything I had installed and seen how long it would take ot beat it (I used this website https://howlongtobeat.com/ ) and im playing the shorter stuff first. I had to kind of force myself at first because ive been really distracted myself lately its a huge issue at times. My main thing was youtube, id just randomly... Source: 6 months ago

I would suggest splitting the short games and long games into two separate lists, order them based on how much you want to play them, and only pick out a long game when you have the time to dedicate to them. howlongtobeat.com is a great resource for figuring that out. Source: 6 months ago

(*3) How many hours does it take to beat main story where applicable. Data from https://howlongtobeat.com - may be inaccurate for games with very few entries. Source: 6 months ago