Are there any Wayland compositor 'toolkits'? 
I'm thinking something that lets you design your own compositor by writing limited code and configuration.
Not locked into a specific paradigm and more capable than just "customize this off-the-shelf WM", but less work than rolling your own with a bunch of libraries.
(Customizable WMs *may* be suitable answers, if you can customize both their appearance and behaviour far enough that a user would not be able to identify the underlying WM from using a customized installation, as an imperfect yardstick.)
@joepie91 smithay might fit your purpose
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@TyberiusPrime Unfortunately that is very much "rolling your own with a bunch of libraries"
@joepie91 sorry. Don't know of anything higher level. The docs suggest it will eventually get there: "The window management and drawing logic is however at this time not provided (but helpers for this are planned for future version)."