That's a tough call. The Caribbean has its own weather patterns that are different than the rest of the country. We're just beginning the dry season in most of the country, but December is the rainiest month of the year in the Caribbean. That stretch I mentioned runs through the cloud forest of Braulio Carrillo National Park, north of San Jose. Cloud forests are misty all year long. It's a twisting, turning sector highway and you'll be sharing it with a lot of logging trucks. It may be okay. It may not.
It gets dark here around 5:30 p.m. all year long. At the very least, you're going to be driving the last 90 minutes in the dark.
What about contacting the Sloth Sanctuary and asking for their suggestions?