What is the optimal sequence of departures and arrivals on a single runway?
Usually a mixed usage runway is more efficient than a dedicated arrival runway(s). Arrivals are harder to get closer together than departures as well. Running a final with 5 mile gaps(for wake and all departures) is a good way to do dep-arr-dep-arr.

With dedicated Dep-Arr runways, and assuming you have aircraft ready to go, your expected hourly rate for Arrival only runways is usually around 30, whereas with a departure only runway the rate is about 45 or so(assuming minimal spacing, and have decent fan headings, and the departure routes are split decently to allow good separation heading to the center).