Hard is one thing, but the endless waves of overly well equipped reinforcements do not make a lot of sense.
That some come to the aid I can understand but by lore, those are loose cells of few people not communicating with each other. How can suddenly so many people show up, ready and fully equipped? From the moment the transport drops down to the moment when the first wave shows up would likely not be more than a minute or such.
On the other hand, I agree, for those people who ask for the difficulty, maybe make it not a fixed number but check for difficulty. Make the 5 turns the new normal (thus less punishing for newcomers or those who want a more relaxed pace... I take slow turns, I hate rushing) and 2 or maybe even 1 for the highest.