We need a compositor that exposes everything as an extension. Preferably in a hot-reloadable, tweakable way, say, using Lua (with JIT). And also exposing its APIs in a way that allows having an analog of xdotool.

Handwaving "just expose an API" ignores the mess at the extension boundary. Modular only works if the contract is airtight, and with Wayland's churn and "sorta spec" documenation, that sounds optimistic at best.

Every "flexible" API turns into a leaky mess unless someone is paid to write the dullest test suite in existance, and nobody is. Mandating one design is ugly, but pretending composition is free is a fairy tale.