You've discovered the reason why everyone offers/completes NVFR training during the winter months! The cloud base is lower and persists longer into the night after a nice warm day, so it becomes really tough to stay above LSALT and also below cloud in these conditions
As mentioned above, head inland or south and you might have better luck, just do it quickly before the heat is
really upon us. I got lucky here in SE QLD and completed it during a cold snap around 4 weeks ago.