One day only at Universal

Hi, we haven’t been to WDW for 10 years, and we are going in early December. We have 10 nights at a WDW hotel, and reserved one day for Kennedy Space Center and one day for Universal. We probably will spend most of our time in the Harry Potter section, that’s what we did at Universal Hollywood because it was so fun and immersive. We know that for Orlando, we have to buy a hopper ticket to see all of Harry Potter. We are taking the Magic Express to and from the airport and will not have our own transportation. Arriving on a Friday, and leaving 11 days later on Monday. Here are my questions:
What day would be best to visit Universal? Based on park hours (want as much time as we can get to see all we want to see), and crowds in early December?
What is the best way to get from WDW to Universal since we won’t have a car – is there a bus transportation? A way to rent a car for one day at WDW or Disney Springs? Or Uber?
Besides the Harry Potter stuff, what is your recommendations for anything else that we really shouldn’t miss?

Thanks so much for anyone who can help with these questions. =)