Interfacelift was my place for a LONG time. Recently they were down, then up, then down, then up/down in so many ways. (https://www.reddit.com/r/interfaceLIFT/). There was still some compression and banding to some of the backgrounds but you can usually find many that are likable and with easy resolution links.
I also find going to https://www.reddit.com/r/EarthPorn/ to be a wonderful place, but resolutions are usually more raw, than a standards for backgrounds. So, finding an excellent picture but not scalable can be sad face, but just going there can be a wonderful journey, and if the picture fits for the background makes it a bonus.
InterfaceLift was pretty much my spot. I've been using the same backgrounds for about 3-4 years. I basically went through and downloaded every 5k nature wallpaper I liked over the course of an hour or so and they just randomly change every hour. I haven't really bothered to change much since then.
I go to Unsplash quite often. There's a dedicated wallpaper section, but even many of the stock photos beyond that can work well for wallpaper if you're so inclined. Many of the pics are 4K or higher, and it's fairly easy to find creators who produce great backdrops (Pawel Czerwinski makes wonderful oil images, for example).