I had a similar issue with maintaining humidity with my cham's enclosure as well. My house is suuuper dry during the winter and it was such a struggle at first. It looks like you've done the basics: shower curtain and a mister. I know you touched on this, but the more live plants you have, and the more coverage, the better. You mentioned that you're waiting for the plants to grow bigger: you could always get some dragon ledges (or build something similar) to mount the plants at different heights within the enclosure. You could also consider getting a humidifier/fogger for your cham's enclosure as well, although some people debate their necessity. I found it particularly helpful to boost humidity at night. I should note that it's important not to keep the humidity too high during the day because that runs the risk of causing a respiratory infection.