I have been in QLD for a month and I am keen to duck out for a day and have a look at that beach run.
With that beach, I am assuming it is the normal 2hrs either side of low tide are the best/easiest time to travel? Or is there more leeway?
Really depends on the tide heights and whether there has been alot of weather and swell. You can check the Rainbow Rocks facebook page for recent photos and comments but I just usually go and check it. If you can't get around safely driving back down to the Freshwater track and over that only adds an hour (give or take) and there is never really that much of a rush to get to Rainbow for anything, except a steak lunch at the pub.
It's also very easy to park your car on the DI side of the rocks and walk up the hill to Rainbow, maybe a 10 minute walk.
Rather than go to Rainbow once you get off the Leisha Track turn right and head out to the Rainbow side of DI........that is a spectacular part of the planet and the true jewel in the crown of the Cooloola National Park.