If you want deep reactivity on the items you would need to wrap the array in$state.
This also splits logic that is coupled, so I probably would not go with this if events or function bindings cannot be used. (You could maybe bundle both variables into a single object, though that might not be the logical thing to do depending on context.)

Note that just using an$effect for this is really is not much of an issue. For certain things, like synchronizing a local, mutable variable from a prop,$effect has actually been recommended for some time (now that$derived can be overwritten, you don't need that any more).

So compared to the$derived approach, I find the$effect code to be cleaner, and as long as you don't create complex dependencies, which could lead to infinite loops, this should not be a problem either.