Paul hi.
There is no way that you will be able to drive into the Booloumba Creek Day Use Area in anything other than a 4x4, as the creek crossing is not a causeway but riverbed pebble, rubble and boulders; plus the steep gravel entry and exit points. You can leave your vehicle the near side of the creek, get your feet wet and walk the 1/2km to the Day Use Area to start the walk!
As you suggested, the best option is to go into Charlie Moreland Campground and take the walking track up to Mt Allan, continue down the far side, where it then cuts the Conondale Range Great Walk. To complete that walk is 56km return and takes 3/4 days.