I’m not familiar with that specific incubator but the style is pretty familiar. You want the vents open during late incubation and hatch, late in incubation and during hatch the chicks need fresh air to breathe. But too much open will lead to humidity loss. I don’t know how you ran the cord for your fan, but make sure that opening is sealed well. Also, I’ve had that kind of humidity problem when I did not get the lid back on correctly with mine. The turner cord was not in the groove properly.
It’s not the depth of water in the reservoir that affects humidity, it’s surface area for the water to evaporate from. To increase humidity you fill more reservoirs. If you spill some water while filling those reservoirs the humidity will spike until the area that got wet dries off. That can make it seem like it won’t hold humidity.
By the way an 80% hatch rate isn’t that bad. Congratulations.
My first hatch of the year is normally in February. There are a lot of people that can give you a lot of reasons to not hatch in winter, and they are all correct. It’s “better” to hatch in the spring or early summer for many reasons. But I hatch eggs and put the chicks straight into my brooder which is built into the coop. They leave the house as soon as they come out of the incubator, even if the outside temperature is below freezing. It’s well above freezing in my brooder.