Help!! Incubating duck eggs!!

[TiaM993]

Hello! I bought a dozen Welsh Harlequin eggs from a local breeder. We are on day 18 now and I only have 6 left in there. I've had 3 that started cracking and were rotting and another 3 that stopped round day 10. I'm pretty sure this is something on my end and I believe it's my humidity. I have a kebonnix incubator and have never had any issues before with chicks. I have noticed that my humidity is either 60 with the vent even opened all the way or I wake up and it's at 30. Does anyone have any tips on how to better regulate my humidity in this incubator? It's just always way up or down. I am buying a govee thermometer to pop in there today, but is there anyway to better regulate all day so I'm not having to fool with it multiple times a day??

Also, for future needs what incubators do y'all like? I've been looking into a chick cozy.