I can see the length you’ve gained. It looks super thick, too. To get straight hair, it is necessary to remove any oils and butters. It takes me a couple of days of washing and air drying to do that. You literally have to strip the hair of those lubricating products. I think if you do that your hair will get straighter and stay straighter longer even if there might be humidity.
Also, straightening in the evening at night allows for two things that straightening in the day does not:
1. In theory it’s cooler than the day and one can control and limit the humidity in the straightening environment.
2. You can straighten then immediately wrap up the warm hair and help it to retain the straightness and make it even straighter with a scarf. The hair is more pliable when it’s warm. As it cools, the hair becomes less pliable and more rigid and more able to accept and hold a straighter style.
Blessed you, you’ve got a gifted Revair which you can use to continue to use and for free.
Great progress on the health, length and thickness of your hair!