Looking for a website for backlog handling and recommendations

https://www.backlog-assassins.net/

I don't know if it can detect your library, but you can input games you liked, and it recommends similar games:
https://steampeek.hu/

Detects you library, then you can filter it based on tags, metacritic or steam review percentages:
https://www.lorenzostanco.com/lab/steam/

Lorenzo's site is great to dig through your own library, as adam already recommended.

But Steam itself actually also has some great tools to find gems in your library or new games to add to it.
The Interactive recommender will use your last and most played games as references for recommendations. You can even unselect games, decide if you are looking for more niche titles etc.

Or you can use one of their tag hubs (like this one for Metroidvanias) and use all kinds of modifications to find exactly the kind of games you are looking for.

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Steam has a recommended for you section which took your played game into account. Aside from that, i recommend blaeo for tracking your played games. If you need a human that recommend you games, join play or pay group; 2 people will pick 7 games for you to play within 3 months (but you need to pay if you fail the bare minimum)

Links:
https://store.steampowered.com/recommended/
https://www.backlog-assassins.net/
https://www.steamgifts.com/discussion/zuIny/play-or-pay-recruitment-come-defeat-your-backlog

Playnite launcher can combine all your PC game libraries in one place. You can install add-ons to Playnite that will suggest what game to play next.

MindGame is an add-on that will ask a few questions and then suggest games from your library based on your answers. Play Next is an add-on that suggests games based on other games you have played, and you can customize how much weight each attribute receives. I use the StartPage add-on customized to show four game categories: recently released, recently added, installed, and a random set of games that I have never opened (with a shuffle button to reroll for a new set of games)

I've found that using the steam's client's own search function with tags and other conditions can be quite practical for narrowing things down from thousands to just a few dozen, then it's much easier to pick one from that list. You do need to have a vague idea of what you're in the mood for and a decent grasp on steam's tags tho.
The recommendation function can also be useful as others already pointed out but in my personal experience it tends to just recommend me games similar to the last few that I've played, so it's just more of the same, not that good for when you wanna switch things around for variety's sake.

In the past I've also used Lorenzo's site but it has so much stuff to toggle and tweak that it can be a bit overwhelming. However it is amazing for when you want very specific sorting of your own library, it can help you to find the perfect game to scratch an itch.

I just wish there was some reliable way of filtering my library so that only games that will for certain run on the PC I'm using at that moment will show up, my laptop is pretty underpowered and just finding which games will run fine can be very time consuming, it makes me just default to using my desktop PC most of the time but sometimes I wanna play something on the couch or bed.

Thanks for the suggestions, everyone! :D
I still can't find the website I used to use and had in mind, but if I find it in the future, I'll definitely post it here!
In the meantime, I'll definitely give your suggested tools a try 😊