Probably Playnite as someone who games a lot. I like to mod my games and get them from different sources so being able to launch Northstar (a launcher for Titanfall 2) or FROST (a total conversion mod for Fallout 4) from one place is nice really nice. You can do a lot of this from within Steam but I find it works a lot smoother in Playnite. You can easily scrape box/cover art for unofficial games, have HowLongToBeat data readily available, have links to the Wikipedia and Nexus Mods pages, and edit the description below the game to say stuff like "Press T to open up trainer menu".

Unfortunately it's not available (natively) on Linux. I've used Lutris but I don't believe it has the same customization options. I don't think there is much in the way of themes besides dark mode and light mode or plugin support. That said I haven't tried to customize it in several years. I've gotten complacent in that aspect and have just been adding them to Steam. I have heard GameHub is another option I have heard about recently but I thought it was mostly the same as Lutris. It turns out it does have some features I was looking for such as popularity scores, game description, and genre tags but I am not sure how the support is for themes and plugins. You can read a decent It'sFOSS article about it here.