wlroots and smithay (I'm not familiar with louvre) do a lot of the difficult work for you, that most compositors will do without much variation but there's still a lot that compositor writers still have to do. It's still a significantly larger task than, for example, writing an X11 WM.

(Well, writing an X11 WM that also includes a built-in compositor is a bit more than just the WM, but I'd say still less than writing a Wayland compositor using wlroots or smithay. For example, xfwm4's compositor is around 5300 lines of C, which is... not nothing, but not crazy either.)