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that's a nice clear definition, you've obviously done the work to inform yourself. im not sure i could have given such a concise definition.
you can definitely migrate away from radix to base ui piecemeal, but for each ui component you migrate, you may need to change every place that component is used because the ref forwarding / asChild prop behaves differently.
in practice, all of the styling presets are customisable so any decision that shadcn made can be overridden. it's just a starting point. and with tailwind + class-variance-authority this becomes a breeze
however if you do modify your shadcn ui components it makes upgrading to the latest versions fiddly because you need to cherrypick your changes and apply them to the updated shadcn component.
How the slider works is a headless component thing (radix in shad). Shad just couples styling presets with the slider which is most of what I don’t like about shad.
Also shad is already beginning to add base ui components too - they live alongside radix. They abstract away components really well, so as shad user you shouldn’t notice the transition.